That's a rather pessimistic statement, don't you think?
I don't agree with you. The Lord of the Flies, though a very good and realistic story, is fiction, nothing more. What would happen if a group of boys was actually stranded in that situation? Who knows? Perhaps the same thing, perhaps something completely different. However, if we're going by that book then you have to look at the facts and see that it was a small group of people that turned most of the kids into monsters. That's how a lot of evil is started. Most people are, sadly, easily persuaded to do things they would never do ordinarily.
There's also the fact that although most of the boys on the island did join the bad side, there were still a couple who did not. Are they still evil? I wouldn't say so.
Now back to nonfiction. Think about a person who sees a complete stranger in danger and runs in to save them without a second thought. Now this same person stole a candy bar from the store when they were a kid, mouthed off to any teacher they didn't like in school, and got into a few fights, one of which ended with this person breaking another person's arm. Are they evil?