Looking for books that relate to....

Are you talking about novels or philosophical treatises?

Sorry, novels.

Well, then, William Golding's Lord of the Flies comes to mind right away. It posits that even children, left to their own devices, will develop an evil streak, quite apart from social pressures.

could you possibly provide an operational definition of "evil"?

I don't know how much use it would be to you but if you want to do the religous bit you could have a look at The Screwtape Letters by CS Lewis. It's basically letters back and forth between the devil and his nephew where his nephew is trying to tempt a mortal to turn evil.

Lord of the Flies is definitely a good idea.
Maybe 1984?
I thought of Animal Farm - yes i know it's about animals, but they're used as an allegory for humans

I'll have a think

Thanks guys.

I dont really have a definition of evil, just evil in the sense that we use it in every day life i guess. Cruelty? Im not sure. Anything you can come up with would be so useful.

try the holy bible, any version.

Dostoievsky's Crime and Punishment...maybe. The main character, Raskolnikoff, is really neither good nor evil. But he commits a brutal murder just because he thinks he is superior to the rest of society and because he deems the victim worth killing. The rest of the book is about his (possible) redemption.