When do you brand someone as "evil". I bet this question has been adequately answered or at least discussed among philosophers. Here is my humble attempt!
Everyday we choose to do certain actions or take certain paths, dictated by certain impulses ...(everyone wants to be rich and happy, and feel pleasure etc....) and we are restricted by others (impulses/motives). You want to eat but you don't want to get fat. You want be rich, but you don't want to steal. You want to act on you rage and kill that person for whatever he did to you ... but you don't do it ... because you are "a good person".
Now it is clear that our actions are influenced by our environment. Teenagers respond to peer pressure to gain a sense of unity and belonging (we all do) . The impoverished are more likely commit crimes in order to get their basic needs or more, and soldiers usually kill on command whether it is justified or not, because that is how they were trained and cultivated!
So when is someone deemed evil?
Its all about pride. When you believe that you deserve more than others who are like you... then you are evil. When you decide that your personal well being or your personal comfort is more important than your peers', you start becoming the villain!
I think there is this internal balance between what we think we are entitled to and what we believe is the right of others!
How much of hero you are or how much of a (war)criminal depends on where you set that balance for yourself.
Philip Zimbardo explains in this unsettling presentation how external influences may turn us into monsters .. and how formidable acts by regular individuals are what define heroes!
Philip Zimbardo shows how people become monsters ... or heroes | Video on TED.com
I must mention however that what I do find unsettling is how he "explains" and and in sense justifies the atrocities done in Abu Ghreib against the Iraqi prisoners.