The Battlestar Galactica finale saw the end of the show’s villain, John Cavil, who died by suicide, bringing the show’s four-season run to an end. After his plan to kill Ellen and obtain the secret to resurrection failed, Cavil decided to shoot himself as his way out of his disgraceful defeat instead of dying at the hands of another. The confusing thing is that there wasn't a resurrection hub anymore for the Cylons, so Cavil ended his life completely.
Cavil, otherwise called Number One, is a fictitious character and the main antagonist in the Battlestar Galactica TV series. The first of eight human Cylon models created after the first Cylon War by the Final Five, Cavil was regarded in the show as a member of the Colonial Clergy, going by the title Brother Cavil. However, unknown to the others, Cavil did not share the Cylon religious beliefs in Battlestar Galactica, often mocking them while pretending to be a proper clergyman.
Cavil’s decision to die by suicide was of the actor’s making. Dean Stockwell, who played Cavil in Battlestar Galactica, decided to stray from the original script, and for a good reason. In the script, Tigh was meant to grab Cavil and throw him over the edge of the upper level, causing him to fall to his death. Stockwell suggested to the show’s creators that Cavil would have realized at that point that the jig was up, and he might as well do it himself.
Why John Cavil Became Battlestar Galactica's Final Cylon Villain
After helping the Final Five create seven more humanoid Cylon models, Cavil grew increasingly jealous of Daniel, the seventh model. He went on to sabotage the Sevens’ genetic code, wiping out the entire line. To complete his evil mission, Cavil locked the Final Five up in a room, suffocating and boxing them before they could resurrect fully. Even after resurrecting them one after the other, Cavil replaced their original memories with false ones.
He further planted them among humans in a bid to teach them a lesson about their perceived foolish love for humans. Cavil's plan backfired as the Five became more sympathetic to the Colonials and their cause, a fate sealed with the destruction of the Resurrection Hub. Without the Resurrection Hub, the Five permanently lost their memories and remained Colonials. This naturally resulted in the Five becoming long-standing enemies with Cavil, making him the last Battlestar Galactica villain and the last one to really fight the Colonials.