Evil is not scientific, nor a phenomenon derived from observation. Evil is a stupid word invented by small minds to explain things they do not understand.
That's what I believe, anyway. I hate the concepts of good and evil, and I think they simplify things way too much for them to be viable to be used in any meaningful discussion.
Hitler was evil? Please. Hitler was changing the world in a way that suited his tastes. A lot of you would do the same thing if you could, yet you always imply that Hitler is the standard for evil, not yourselves. There are many other words we could describe Hitler with: selfish, intolerant, extreme, severe, biased, prejudiced. Any of them work and explain things so much better than just shutting down our brains, taking out the label of "evil" and slapping it on without any other thought.
The word "evil" is very general and doesn't have a specific definition. That's why it can so often be misused, and that's why you will never understand anything or anyone for whom you only use the word "evil" as description. It's a childish term, really, and I cringe every time I play a game and the only thing I hear about the enemy faction is that "they're evil".