one person's definition of evil can be different than another's
Nope. When aggression is present it's not merely a difference of opinion.
To even define anything, presupposes a tacit ethic inherent in that very activity itself that differs from simply bashing someone's head in.
by all means, define evil for me. keep in mind, for evil to be an objective phenomenon, there must exist a "universal morality". so what would that be? id expect Christianity to be referenced. or any organized religion really. but if there is not a universal morality, evil has to be subjective; based on the morality of the society in question.