As a serious answer, I think the community as a whole is seemingly monstrous for a few reasons.
The biggest one, I think, is the lack of real graphics. Think on it this way; if DF was ultra-realistic 3D, do you think everyone would be quite so willing to pull a magma lever and watch as a mass of people melted horribly right in their faces? Or would Adventure Mode players be so earnest in tearing the limbs off an elf and then crack his head open with it, watching as the guts and brains oozed out? I sort of doubt it.
DF has opportunities for this sort of terrifyingly awful stuff, but the reason we aren't totally disgusted and horrified is because it's all in ASCII. A kitten being systematically taken apart for meat is probably an incredibly disquieting and violent thing to watch, but around here it's practically standard protocol.
I don't think it's because everyone around here is a hideous person that is just full of sadism, more like we see this sort of thing happening onscreen and it's just so over-the-top ridiculous, it comes across as humorous. And it is. Kitten biscuits feeding an entire city of people, seeing your bookkeeper go nuts and claw a goblin's eyes out, etc. The community has sort of cultivated this culture of crazy unnecessary grotesque violence for comedic value.
And it's okay, since it's not like we're slaughtering real kittens or anything. It's just a game, after all.
Anyway, that's my theory. It's probably dumb and wrong, but take it for what you will.