I don't mean "mentally ill" in the same frame of reference as someone who is experiencing mental illness, so it's a poor choice of words on my part. There is something wrong with people who choose to do these things - whether it is a lack of morality, vengeance, little man syndrome, whatever - the point is something isn't working correctly in their minds to the point where they either don't realize the inherent evilness of their actions, or they think it is a right course of action.