Jack Montgomery was the main antagonist of the Supernatural episode "Metamorphosis". He is a normal human, but contains a dormant gene in his DNA capable of turning him into something called a "rugaru" - a cannibalistic monster-like creature. These genes activate near the end of the episode.
Jack begins to notices the changes in his behavior when he notices that he has an insatiable hunger that he cannot seem to get under control, going so far as to eat raw meat, and his bones shart shifting around under his skin.
Sam and Dean Winchester get a call from their friend, a fellow hunter named Travis, who has also been observing Jack. Travis tells the brothers that he has identified Jack as a rugaru. He goes on to explain that the rugaru gene was genetic, and Jack inherited it from his father, whom Travis had killed back in 1978. Travis also explains that, the moment Jack gives in to the craving to eat human flesh, his full transformation into a rugaru will become complete and permanent.
Meanwhile, Jack begins noticing more changes in himself when he gets urges to eat his own wife when she cuts her hand accidently (the sight of her blood giving him a craving), later comes on to her rather aggressively (which she does not like) and comes dangerously close to devouring a young woman whom he sees jogging - but he manages to resist these temptations, avoiding his transformation into a rugaru.
However, when Travis goes to Jack's home with the intent to kill his wife (when it is discovered that she was pregnant; Travis was doing it to prevent the birth of another rugaru) he snaps and fully transforms, biting a chunk out of Travis' neck, which causes his wife to flee in terror.
Jack eats Travis, and knocks out both Sam and Dean when they arrive to check in on him, locking Sam in a closet. Sam tries to convince him that his actions are more important than his nature, but Jack is overwhelmed with despair and hunger, and is fighting not to eat Dean, who is still unconscious. Before he can, Sam picks the lock, breaks out and sets Jack on fire, killing him.