Why do some people seem to be naturally and wholly evil?

we all stem from the same place. so why do some people become so naturally vile, seemingly out just to make others feel negative emotions? of course a person can become this way through lack of love and nurturing.. or through being attacked, they would rather be attacking, than be attacked. these are all i can come up with in the way of answers to this question. anyone else have anything to suggest? much love :)