Brokenstar's Bitterness

What made this cat the murderer he's known as?

Brokenkit is a ShadowClan cat through and through, yet he doesn't know the identity of his real mother. He is mistreated by his foster family, and he can't wait to become a fearless warrior.
As a warrior, Brokentail's life turns into a quest for glory and power. His reign as leader was one of the bloodiest in ShadowClan history, which ultimately led to his downfall.