Oddly enough, I'm gonna have to go with Kind. I mean, the fact that I find it difficult to write villains without accidentally making them nice at some point is proof enough. ^^;
If I had to reason through it, though...it's just that I find cruelty difficult to understand on a personal level. Like, if I had to choose between seeing someone I hate suffer and seeing them disappear, I'd definitely choose to just never see them again.
As much fun as it is to get revenge or watch karma in action, I greatly prefer simply not having to deal with a problematic person at all. So it's hard for me to get myself into the head of a character who prefers the opposite...
That isn't to say no one is ever cruel in my stories (far from it...). But that "cruelty" is, now that I think about it, often simply the result of misunderstanding and indifference.
A godlike entity plays with the character's lives because they just don't think their suffering is worth caring about. Or a ruthless character destroys everyone around them not out of malice, but selfish self-preservation. I very rarely write someone who is fully aware of who they're hurting and how and why.
I do have a few sadists who kinda fit the bill, but sadistic characters are often closer to monsters than to actual people...it's just 'hurt people'+'have fun'; you don't have to think too hard about it. Not like you do when you create a truly vengeful character...a flat sadist can work in many contexts; a flat vengeful character is just a (bad) cartoon villain.
All that is to say, although kind characters are easier for me to write, I spend much more time on the cruel ones; they're like a welcome challenge~.
It is nice to take a break and just write a cinnamon roll every now and then, though. One such character inspired this thread: I look forward to all of his scenes; he's just so relaxing to write for, and I've finally figured out why. ^^