I don't know, when we used to play MUDs (precursor of MMOs), I had some buddies who would cheat/break the rules because it was fun. They were very skilled gamers and had legit characters and played seriously for the most part. But they'd get a kick out of cheating and pissing people off. The more you complain and rage, the harder they laughed. Seems about how most bully/victim relationships work.

I imagine others who cheat are skilled players who just get bored and start cheating for fun. They can't get away from playing the game, but they don't enjoy playing it as seriously anymore.