Depending on your source / religious belief, evil has been a topic that people tried to explain for millennia. As usual, whatever the religion of your particular people, you are right and everyone else is wrong.
You can get a topic overview here, and expand the picture if you want:
This article surveys the way world religions understand the problem of evil, its origin, nature and end.
www.comparativereligion.com
The bottom line regarding "evil" is that it's a biological / psychological "glitch." The idea is, regarding the brain, using the best example, is that non-empathic people have no feelings for others. They have no feelings because the areas of the brain designed to create "feelings," simply put, don't work. This would be the foundation of potential problems, and if pushed over the line - watch out.
In this picture, the comparison between, for instance, sociopaths and psychopaths make a good defining picture. If a sociopath has had a good / non-intrusive life from parents and people while growing up, they will simply grow up and do what needs to be done career-wise, and although they aren't 100% "hidden" ... they still lead a basically quiet life. I've known these type people for years, and they're interesting to watch and study.
This idea is best defined in this video.