Peter Hale, Derek Hale's uncle and one of the Hale werewolves, reveals in Season One that he killed Laura Hale (Derek's sister), who was the Alpha, or pack master at the time. As the new Alpha, Peter bit Scott McCall, and used his new-found strength to brutally murder everyone involved in the arson of the Hale house (which killed 10 Hale family members, including many innocent non-werewolf humans). His killing spree ended with Kate Argent (Allison Argent's aunt, Derek Hale's former girlfriend, and the mastermind behind the the Hale house arson). Peter was wounded by Jackson and Stiles, and ultimately killed by Derek to avenge Laura's death. Killing Peter made Derek the Alpha. Peter manipulated Lydia (who was bitten by Peter and proven to be immune to the bite ) into resurrecting him in Season Two, bring him back to life but greatly reducing his strength and fighting ability. He is still very cunning and crafty. He regains fragments of Derek's trust and helps him out periodically, but his real loyalty is often in question.