This is another question I have often pondered. What
is evil? As a full time GM of role playing games I often must portray good and evil convincingly to the players, and in a game like D&D or Pathfinder the difference makes a big difference. But I often find in writing complex stories for the players to participate in, that the definition of "evil" depends on the perceptions of the player. I can demonstrate by asking the age old D&D question "So, what do you do with the baby orcs?" If you kill them while they are helpless babies, is that evil? If you spare them and they grow up to attack innocent people, does that justify killing the next bunch of baby orcs? The same kind of thinking can be applied to real life. But I confess this is merely academic, because the reality of life is much more black and white than art would have us believe.
One thing I can say from personal experience is that some human beings simply do not feel anything resembling common decency and humanity. Perhaps that, in it's fullness, is really the definition of evil.
"Luke, you're going to find that many of the truths we cling to depend greatly on our own point of view." -Master Obi Wan
I have always hated that line. One should be extremely suspicious of a ghost telling one that everything he knows to be true is a lie, and almost in the same breath who one's greatest enemy really is. The truth, my dear Obi-Wan, is not quite so flexible!
Scott