This is the Black Hood. I am the man who shot the adulterer at Pop's. I killed the child predator in Greendale. I shot the drug and sex addicted teenagers at Lovers' Lane. Riverdale is not innocent. It's a town of hypocrites, degenerates, and criminals. My wrath is the price of your lies, your secrets, your sins. I will not stop. I cannot be stopped. I am the wolf. You are the flock. This is the bloodletting. You will hear from me again.
All of the Black Hood's victims are or at one point were residents on the north side of Riverdale, and/or have committed some sin, such as being a child predator, committed by Geraldine Grundy, sex and drugs, committed by Midge Klump and Moose Mason, adultery, committed by Fred Andrews, and drug dealing, committed by Robert Phillips. This has caused Polly Cooper to leave town out of fear of being that Black Hood could target her, due being an unwed mother, and being pregnant with her own cousin's children.
In "Chapter Thirty-One: A Night to Remember", during "Carrie the Musical", it was revealed that the Black Hood may still be alive after several threatening messages were sent to Kevin Keller. These letters were signed by the Black Hood and demanded the role of Carrie White (played by Cheryl Blossom) be recasted. When it came time to perform the musical, the body of Midge Klump, who replaced Cheryl as the role of Carrie, was discovered on stage, stabbed and crucified, announcing the Black Hood's return.
In "Chapter Thirty-Four: Judgment Night", it is revealed that Hal Cooper is the real Black Hood and that Joseph Conway was merely his pawn. Betty's speech at the 75th Anniversary Jubilee reminded him of a promise he made to his mother that Riverdale must and will do better. However, there is still another Black Hood on the loose after having attacked the town hall (while Hal was there), and attempted to shoot Fred Andrews again (although Hal was at home).
The morning following the 75th Anniversary Jubilee, an event that was supposed to unite Riverdale, a masked gunman entered Pop's Chock'lit Shoppe. He stood on top of the counter with a gun pointed to Pop Tate, making demands. While doing so, Fred Andrews looked over at his son, Archie, and told his son to stay back. The gunman then turned his attention to Fred and demanded his wallet at gunpoint. Archie panicked and ran out in between the gunman and his father. Despite Archie's frantic decision, Fred was shot in the stomach. After firing that single shot, the gunman ran out of the shop, leaving a critically injured Fred for dead.
Late one night, he broke into GeraldineGrundy's house after she sent Ben, a music student, home from piano lessons. After she bid him goodbye, she realized her living room was in disarray. With her back turned to the Black Hood creeping upon her, he strangled her to death with a cello bow.
At night, in the woods of Riverdale, he found Midge Klump and Moose Mason using the new Jingle Jangle drug given to them by Reggie Mantle, and making out in their car, about to initiate sex. He walked up to their car window and quickly fired three shots at them with the intention of killing the two teenagers, before walking away from the scene.
Acknowledging Fred Andrews's recovering, and Moose and Midge surviving the shooting (Moose ended up in the hospital, due to him being Midge's human shield), the Black Hood wrote an open letter to the town, and sent it to Alice Cooper as a message to be published on The Register as a warning article about his intentions, affirming his presence in town as a vigilante hunting for what he deems to be sinners. He also promised to send more letters. 
The Black Hood sent a second letter to Betty Cooper, revealing that her speech at the 75th Anniversary Jubilee had inspired him in targeting sinners. Attached to the letter was an encrypted message about the location of his next crime, which Betty later deduced to be at the town hall during a meeting. Fortunately, she had the meeting evacuated before the Black Hood could launch an attack.
Close and Closer
The same night, while Betty was on her bed, he anonymously called Betty on her smartphone to have a talk with her, informing her that this was the Black Hood speaking.
Speaking at the phone, the Black Hood revealed to Betty that he was at the Town Hall and had not attacked anyone because that night was all about Betty. He claimed he had the same goal of Betty: setting Riverdale on the righteous path again and cleanse it of sinners. He then informed Betty that knows that Polly is staying at a barn two miles out of town, and threatened to end Polly's life if Betty spoke with someone about their secret conversation to anyone, or warned the police.
The Black Hood called again later, while Betty was with Archie in her bedroom. The Black Hood asked Betty to publish a defamatory article on her mom, with the promise to reveal information about him to Betty in exchange. Since Betty lived up to her part of the deal, the Black Hood replied to her question, stating that Betty would recognize the face beneath the hood. Shortly after, the Black Hood ordered Betty to break her relationships with Veronica and Jughead, threatening her if she had not obeyed.
After Betty had done what he requested, the Black Hood lured Betty into an abandoned home where she found a gift box in an empty room; Once opened, Betty discovered a hood that the Black Hood ordered her to wear through a conversation on the phone. He then asked her to look herself in a mirror to see how similar they were.
Later that night, the Black Hood called again, saying there was an unfinished business to clarify. He revealed that he had watched Betty very carefully and knows that she shared their secret with Archie, which he is not happy about. The killer threatened to kill Polly and the rest of Betty's family unless she named another sinner for the killer to unleash his anger on. After an initial reticence, Betty said to kill Nick St. Clair, who had earlier attempted to rape Cheryl Blossom, and gave Black Hood his location at the Five Seasons. Impressed, Black Hood told Betty that they were the same, before hanging up. 
The next morning, feeling guilty over choosing Nick to die, Betty rushed to the Five Seasons, hoping to save Nick. However, when she arrived, Nick was alive and well, but beat up, and Nick says if she's here to finish off what the Pussycats started. Betty was confused and went outside to call the Black Hood, asking him why he didn't kill Nick. He explained that he didn't kill Nick due to him not being a son of Riverdale, meaning he wasn't a resident, and also he just wanted to see Betty's true nature, and the real work can begin.
The Black Hood called Betty again to get her help to find her next victim, The Sugarman, who has been supplying the Jingle Jangle drugs in Riverdale. He assigned her to find out who the Sugerman was, and eventually, Betty did so with help from Cheryl. However, when the Black Hood asked for the name, Betty informed him that she already told the Sheriff about the true identity of the Sugarman. She had published that on the Blue and Gold, where she exposed Robert Phillips as the Sugarman. Unhappy with Betty choosing justice instead of execution, the Black Hood informed her she was playing a risky game. Betty agreed but countered that it was her game. Since she found out who killed Jason Blossom, and who the Sugerman was, Betty vowed to catch the Black Hood next. Later, the Black Hood snuck into the jail where Phillips was being kept, confronting him at his jail cell at gunpoint with the intention of killing the drug dealer.
The Black Hood appeared in Josie's stress-induced nightmare. In the music room of Riverdale High School, he slowly approached her from behind as she rehearsed one of her original songs, slashing her throat open with a knife.
The Man Behind the Hood
In Betty's dream, the Black Hood appeared in the Cooper house, disguised in a Santa Claus costume as he placed gifts around the Christmas tree. After killing, Alice, Hal and Polly, he slowly inched towards Betty, crawling over the living room couch with a knife in hand. With fear in her eyes, Betty stood there without response until waking from her sleep.
After kidnapping Joseph Svenson, the Black Hood cut off his finger and sent it to the Cooper residences under the pretense that it was a Secret Santa gift for Betty. After she had opened his gift, the Black Hood called Betty, telling her that Svenson had lost a lot of blood, though it was nothing compared to the blood that was on the hands of Riverdale. He instructed her to exhume the past in order to discover the origin of the primal sin. Should she do so in a timely fashion, she might find Svenson alive. Just as he was to inform her not to tell Sheriff Keller, Archie grabbed the phone to talk to him directly, forcing the killer to end the call.
Archie and Betty first visited the Sisters of Quiet Mercy, where Svenson lived temporarily, following the murder of his family. There, they discovered that Svenson's primal sin was sentencing the wrong man to death. The preacher who he had identified as the Riverdale Reaper, was not his family's killer. Their trip to the group home then led them to Rose Blossom, who was involved with the group of men that executed the innocent preacher. From her, Archie and Betty learned that Betty's grandfather not only participated in the execution but that they buried the preacher alive at Pickens park. The Black Hood presumably watched from afar as the teens dug up the shallow grave labeled "Here Lies Joseph Conway – Sinner". Upon opening the casket, they discovered that it was empty. Their concern to wonder why the Black Hood would take the time to bury an empty coffin wouldn't go unanswered for long. He showed up soon thereafter, holding the teens at gunpoint as he ordered Archie into the empty coffin and told Betty to bury him with dirt. As she did so, police sirens began to ring throughout the woods. The Black Hood became distracted, allowing Betty to take advantage of the opportunity by hitting him in the face with a shovel. Wounded and disarmed, the Black Hood took off into the night, leaving his gun behind.
He ran onto the bridge which stood over Sweetwater River. The Black Hood came to a stop as the teens managed to catch up to him. In a reversal of roles, Archie had him at gunpoint, with his own gun nonetheless. As he attempted to jump over the edge and into the water below, Archie shouted at him to get down or else he would be shot. Archie took the time to confront the Black Hood on his previous attacks (i.e. shooting Fred Andrews, Moose Mason and Midge Klump). He would indeed pay for his crime. As far as Archie was concerned, there was no other option. Despite being warned, the Black Hood attempted to jump, though unsuccessful as he was shot down and killed by Sheriff Keller. After instructing Archie to put the gun down, the Sheriff removed the hood to discover that the killer was none other than Joseph Svenson, who had gone as far as removing his own finger in an effort to further the ruse that he was merely a victim in the Black Hood's twisted games. As for why Svenson chose to murder the residents of Riverdale, Archie, Betty, Jughead and Veronica concluded that targeting sinners was Svenson's way of balancing the scales for his primal sin and that he was getting revenge on the town he blamed for the death of his family.
Following Riverdale High's announcement of its spring production, "Carrie: The Musical" and Cheryl Blossom's casting as Carrie White, someone claiming to be the Black Hood sent Kevin Keller a letter demanding that the role of Carrie be recasted. When Kevin failed to comply, he sent Kevin another letter, claiming responsibility for the sand bag that fell and nearly crushed Cheryl during rehearsal. Should Kevin fail to heed his warning, the Black Hood claims that the sand bag won't miss next time. Kevin does as instructed of him by removing Cheryl from the play. Her understudy was Midge Klump, who the Black Hood killed on opening night. Midge was crucified and staged on set for the entire auditorium to see. On the wall, written in blood was a message. "I Am Back From The Dead. All Those Who Escaped Me Before Will Die... B.H."
Black Hood calls Betty. He asks if Betty missed him. Deep down, she knew it wasn't over. Betty accuses him of killing Midge and setting up Svenson. The Black Hood admits that both is true. And yet, for all he's taught her, she's been making amateur mistakes by allowing Chic, a sinner in her house. The Black Hood claims that Chic has lived in a den of sinners and that he kills as easily as he did in the house. Chic has supposedly murdered before.
When the Black Hood calls again later, Betty admits that he was right about Chic being a killer, but he won't admit it. Some sinners only confess when faced with certain death. If Betty delivers Chic to the Black Hood, no one will ever know about the man who met such a gruesome end in their living room. Betty seems to be considering the Black Hood's offer, but before she can answer, FP and Alice barge into the house. The Black Hood meets Betty and Chic at a cemetery. Chic doesn't believe that Betty is truly offering him to the Black Hood as she too would be running for her life if it was really him. But Betty tells Chic that the Black Hood is only interested in him. She lowers her gun and gives Chic a head start as the Black Hood slowly approaches. Chic takes off in the opposite direction. The Black Hood briefly makes eye contact with Betty before chasing after Chic.
The night of the Mayoral debate between Fred Andrews and Hermione Lodge has arrived. The Black Hood bypasses security for the event by entering through a trap door leading from the balcony to the roof, a layout that isn't on any building plans. From the balcony, the Black Hood begins shooting with a rifle. His first target is Fred, but he misses, though he fires several more rounds before leaving the event. He later stops by Thistlehouse. When Cheryl answers the door, she screams in fear. 
Unmasked and Defeated... Or not?
Cheryl screamed and ran to her bedroom, where she locked herself in. The Black Hood slashed down the door with an ax, but not before Cheryl managed to escape through the window. He followed her outside the house, finding Cheryl in a hunting cape with a bow and arrow. After he was shot in the shoulder, he fleed into the woods. Cheryl called Betty and informed her of the situation.
FP called Betty and informed her that Hal was at the hospital. She arrived at the hospital to find Dr. Masters dead in her father‘s bed. The Black Hood called her and told her to return home or he will hurt Alice.
At the Cooper house, Betty found Alice and Hal sitting in the living room, preparing to watch some old home movies. Hal showed them a clip of himself as a young boy promising to keep Joseph Svenson's mouth shut about his family’s murders, which were performed by Hal’s father, Louis Cooper a.k.a. The Riverdale Reaper. Hal then enticed Betty tray expose him on tape. She revealed that he is the Black Hood, listing all of the murders he did, though Hal did admit the attack on the debate was not his doing. As he tried to strangle Alice after she successfully managed to trigger him in order to free themselves, Betty hit him over the head with a poker, knocking him out. Hal was arrested by Sheriff Keller in front of curious neighbors gathered on the sidewalks.
Earlier that evening, however, another Black Hood attacked Fred and Archie. He attempted to kill Fred again, but Fred was wearing a bulletproof vest that saved his life. Archie then chased the Black Hood outside the house, but he had already vanished into the night, later realizing that this Black Hood could not have been Hal.
This masked assailant is seen to be a tall man who appears to be 5'10" tall per Archie's testimony to the Sheriff. He has green eyes as Archie described to the cops, and as was seen. He wears a black hood, hence his nickname.
Editor's Note: The list of suspects detailed below are of characters who have been suspected within the show of being the Black Hood. It should in no way list characters who fans have deemed as being potential suspects.
Hiram Lodge / Hermione Lodge (by Veronica; Disproved in Hermione's case since Black Hood is male; Disproved in Hiram's case as Hiram hid behind the podium with Veronica and Hermione when the Black Hood attacked during the Mayoral debate)
He is based on the character from the 1940, 1943, and 2015 MLJ/Archie Comics, "Black Hood", who is an ex-cop turned vigilante. Black Hood is known for wearing a black hood with eyes holes cut-out and mainly kills people whom he believes are criminals or are involved in criminal activity somehow. 
According to Roberto Aguirre-Sacasa, Riverdale's Black Hood share some similarities with real-life Zodiac Killer, an anonymous criminal active in northern California in the late 1960s and early 1970s that has never been caught. Both killers targeted and shot couples on love lanes, used different weaponry, and sent letters and ciphered messages to the press.