Writing Evil

My current wip has a true antagonist. When this villain arrived in my story, I was thrilled. I've never had a real bad guy character before. I thought it was going to be fun. I planned and anticipated his dastardly acts and despicable deeds. He would make life impossible for my protagonists.

True confession time: Writing a believable bad guy is HARD.

What is evil? I would like to think that I am not a bad person, so am I capable of getting into the mindset of someone truly bad?

When I was a child, a horrible man lived in my neighborhood. He had a wife who died in a hot tub accident, and later, after he remarried, his new wife and stepson were killed in a boating accident. The second accident turned out not to be an accident (and perhaps the first as well). He killed his wife and stepson to collect insurance money.

I spent many hours thinking about how someone could be so horrible and detached to kill for money. No one had a clue. He was a very polite and gentle man. One day, in a time period between the two incidents, my dog bit him. I suspect she knew he was evil. The rest of us did not.

So, the challenge, how do I reveal evil below the surface? How can I make my villain truly scary?
I may not be a perfect person, but I don't understand evil.

I'm just glad my evil character is a creation, not a real man.