I don't think he was evil. he was misguided. he was manipulated by Luthor (and possibly Psycho Pirate).
as far as the fight, think about it, he's essentially got the power level of a teenage Silver Age Superman, but with no training. he doesn't know how to pull his punches, so he's knocking people's heads off. in his state of mind, them attacking him causes him to attack, therefore kill, making him evil.
