I guess it it is important first of all to see evil and good as a continuum rather than a static alignment. For realism it is necessary to explore the character's motivations for the way he behaves and then carefully detail the event(s) that lead him/her to change his outlook on his/her own behavior. No one wakes up and says "I think I' going to be evil today" and very often, those who are percieved by others as evil see themselves as justified and seek to explain the necessity or extenuating circumstances leading to their "evil" behavior.