A couple of people brought this one up as a possible subject:
Doing mean things to your characters--how do you bring yourself to it? You would never kick a puppy or arrange a lethal trap for a pack of heroes, but your villain does, and loves it. How do you get into the mindset of your darker characters, and deal with the consequences?
There are some writers who really, really get into just this sort of thing. In fact, for a long time there was a subset of fannish writing called "character torture" that spun off from the pon farr part of Trek, mixed in with some of the brutality Dorothy Dunnett put Lymond through, especially in Disorderly Knight6s. This "let's really torture our cute heroes and have them suffer charmingly" had another parallel story type called "hurt comfort." It goes way back--I remember someone heavily into Tolkien fanfic in the sixties, who wrote a whole series of stories based on Frodo being tortured by orcs and various other Mordorian nasties and Sam comforting him. Poor old Frodo went on to have a series of very painful adventures, with Sam comforting him back to life each time. The writer adored Frodo--in fact, she went by the name 'Fodo' briefly, until another person showed up who also went by the nickname 'Frodo.' Oh, and a third, a guy this time, from another country, a few years later, was also Frodo.
Anyway.
This pain/pleasure thing is probably akin to the sex-and-pain thing we were talking about in the vampire discussion. Or maybe not? There was one some discussion on this subject on GEnie, but I forget if anyone came to conclusions to which all agreed.
Others are squicked by that whole subject. They know that bad stuff has to happen to characters, and they also know that irredeemably bad Dark Lords can be really, really boring. Dark Lords and Evil Emperors are pretty much interchangeable because they are predictable. If a villain is always going to do the worst thing possible, you can pretty much skip ahead until the defeat at the end, unless of course you get into all those torture-and-kill-and-rape scenes.
For a while a year or so back, all over the writerly portions of the Net everyone seemed to be riffing about how villains are the heroes of their own stories. We all agree that villains have motivations and justifications that seem reasonable and even right to them. Some writers extrapolate that out to equate morality with self-justification and ethics with convenience, others (like me) prefer to search for moral verities, even if they aren't easy to define.
Then there are the villains who have built justifications that seem logical and convincing to them...but to everyone else look as mad as a bag of snakes. Getting into the shoes of such a person, and seeing the world through their eyes, can be really, really creepy. So creepy, in fact, a writer can look down at the words just written, appalled, and wonder where that came from, and does it mean I'm a rotten person? Are people going to read this and think I'm like that?
The answer is, yes, some readers can be counted on to put two and two together to make twenty-two, and go leaping and bounding to conclusions about your personal convictions. You will find yourself roundly condemned for your villain's wickedness. But that's later, when someone else reads it. Right now you're facing that just-written page and thinking, should I water this down? Should I get rid of that scene? Why is it there, do I really need to have that in this story?
Inevitably some reader is going to slang you for 'gratuitous' X, but do you believe it's gratuitous? You might wrestle mentally over that question for days--for months--and some will offer you comfort by saying, "If you're struggling, that's good! That's what we need, more uncomfortable stuff out there, it makes people think!" And you're nodding and smiling, but at midnight you sit there wide awake saying to yourself, "Makes them think about what? What am I really doing here? Am I just perpetrating more of the pain and evil that the world is already overflowing with?"