I just killed my first dwarf today (deliberately, anyway) and that wasn't fun. He was a tax collector that liked adamantium in a non-adamantium-bearing region and he was having my metalcrafter repeatedly hammered for failing to produce some, so I really don't feel that bad about it, but I still tried to do it in as businesslike a manner as possible. Unfortunately, never having done this before, the spear trap I set up to get him with only wounded him. With yet another mandate winding down and the metalcrafter unconscious with a mangled leg already, all I could think of was to lock the tax collector's room and let him die of thirst. Ick.
On the other hand, I'm not nearly so sympathetic to goblins. I figure if they're going to start the fight on their initiative, coming to my land to murder my workers and kidnap my children, they should be prepared to accept me going all Vlad the Impaler on them. I'd love to be able to construct skull totems as furnishings, I'd line the roads with 'em as a way to show the goblins the score.
Elves are an interesting middle ground for me right now. They don't do anything nasty to my dwarves directly, they're just tremendously annoying. But now that I'm paying more attention to worldgen, seeing all those ruined cities on the map that the elves have eaten their way through, I find myself wanting to pick a fight. Even so, I'd like it to be a 'civilized' war (as much as it can be). I just spent a lot of effort caging the elven ambassador rather than killing him because I hope to release him once the war's over, for example. (I know I can't 'really' do that, he promptly went berserk in the cage, but it's a game so play along