“How do you define evil? “
Evil is someone or something that harbors anger, selfishness and too much ego.
“How do you know an enemy? “
You can't really, until that said enemy presents an obstacle for you or others to overcome.
“What predisposes one to good or evil?”
Anything can. It depends on how that person takes it. If someone told them to have a good day, and they were already having a bad day, they would take that for sarcasm and their day would become worse. It all depends on the persons state of mine.
“Is it genetic or environmental?”
I don't believe anything is genetic {except for some genetic diseases}. I believe everything is environmental. If your parents hate a certain brand of underwear, for their own reason, you'll grow up disliking the same brand - or harboring something against them. Your environment causes you to be the way you are. Of course, you can always break out of your environmental patterns, but that's another paragraph ^__^
“Can a choice be made by the individual (monster / NPC)?”
To be good or evil? Yes, I believe that some people, early on, can let themselves succumb to evil or good. Depending on what feels right for them. Also, they could just beat down their conscience enough to become evil. It all depends, but they made that choice.