All my current players are, in theory at least, playing Good characters. Apart from taking a wholly unseemly pleasure in chopping other sentient creatures into gobbets of meat and stealing all their stuff, they generally stay more or less within the lines.
That means that Being Evil is up to me. I'm the GM. I get to play all the Bad Guys.
The trouble is, I'm not very evil. I hate evil people. They really piss me off. Also, it makes me feel distinctly uncomfortable to pretend to be evil.... which is a bit of a problem when most of your job consists of Being Evil.
I'm not talking about muahahahahah!!! hand-wringing dastardly-plan Adam-West-Batman-villain Cartoon Evil here; most of those guys barely make it up to annoyingly naughty. Their so-called Evil tends to be at about the level of a sniggering schoolboy slipping a fart-cushion on to the teacher's chair, and so presents no particular moral or ethical problems for me.
Huge-scale institutional Evil isn't that much of a problem for me to GM either, because it's all so impersonal and regimented. Hitler, Pol Pot, Stalin were all undeniably evil (not to mention completely fucking insane), and they instituted some very unpleasant regimes, but I don't generally feel any immediate personal ickiness about GMing that sort of thing. I'm removed from it; it's all just bureacracy with jackboots.
When it comes to your individual evil shit-bag though, the kind of guy who will expoit people face-to-face, who will go out of his or her way to make people's lives more miserable, who will torture and murder both for the fun of it and because it will make other people sickened and afraid.... I have a real problem GMing those arseholes. It may all be make-believe, but I still have to get enough into that mind-set to come up with a believable course of action for my little imaginary psychopaths.
I don't like it. It gives me nausea of the soul to think like that, even peripherally, so I tend to wimp out and go with adversaries who may be savage and brutal killers, who may very well hunt down your children for the purpose of feeding them to their children — but at least they have some sort of rational reason for acting that way.
A side-effect of this attitude is that there's a lot less capital-E Evil around in my campaigns than in a lot of others, a fact that (I think) is tending to piss off the paladin(s) in the party a bit since it makes their Evil-o-Vision™ rather less frequently useful than it might be. Orcs may have a savage, brutal culture, and goblins may be sneaky, treacherous little shits, but they're not Evil. Bad, yes. Evil, no. Maybe some of them are Evil. OK, almost certainly.
Anyway, one of the reasons I like to be able to take a break from GMing every once in a while is because just once in a while, it's nice to be the Good Guy. Being Evil actually kind of sucks.