Why Are There Evil People?

And what makes them that way?
I’m not familiar enough with the teachings of other religions, but at least as far as the Christian religion is concerned, this question is as old as, well, as old as Christianity. If the One God is omnipotent, why does he allow evil, when he could wipe it out singlehandedly?
It made a lot more sense to me to have a variety of gods (a la the Greeks, Romans, and Scandinavians) who had virtually equal powers. Some days the good guys won, some days they lost. It seems to me you get into a lot of philosophical trouble with the whole monotheism thing. I can’t figure out how it ever caught on.
In our lifetimes, I suggest that Adolf Hitler is what we consider the personification of Evil, although there are many other evil people in the past who were at least as bad as he was.
The explanation I most often hear is, God gave people free will so they could choose good or evil. But wait. Why is there evil at all? It seems like the choice might have been between good and less good.
The other answer, which is a little easier to swallow, is “I don’t know”. God is inscrutable and beyond our understanding, so we can’t begin to question the whole thing. Well why not? Why didn’t God make it easier for us dumb critters to understand?
Evil, I think, is a little like U.S. Supreme Court Justice Potter Stewart described pornography: “I know it when I see it”.
I think of it as trying to hurt animals or people who are weaker than you and can’t fight back, or trying to hurt someone for personal gain.
This is the exact scenario I’m going through at work, with a person who literally began a campaign to get me fired, hoping she would replace me. I’m not imagining that; she said it outright. She has read my email, gone through my desk, taken pictures of me secretly with her cell phone, accused me of being a thief, and had numerous conversations with other employees to try to win them over to her “side”. In some cases it’s worked; in others, it hasn’t.
I am mystified as to why she still works for my company. And I am hurt, and shocked. All I ever did was try to help her succeed. And I feel like a complete idiot for trusting her. I will never understand it ever, but I do know this: she’s evil. I know it when I see it.