This is Sparrowpaw. His father is the leader of his clan, Leafclan. His father is also very evil. He expects his son to grow up to be just like him. But Sparrowpaw wants a normal life as a warrior. His father hates this, and trains him to be as evil as he can be. The other cats realize this, and try to help him. His best friend offers to help, and Sparrowpaw lashes out at him, killing him in a single blow. He realizes what he did, and runs away into the forest. Now he lives as a loner, hiding from his father....
This is the second in the 'Fighting The Evil Within' series.