If dwarves have no qualms with killing elves in cold blood, then they shouldn't have a problem with slavery. But they should only be able to enslave invaders, which would be especially relevant with the army arc.
You're forgetting that dwarves actually get along with elves quite well in most cases.
[ETHIC:KILL_NEUTRAL:ONLY_IF_SANCTIONED]
Also, while I'm at it,
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:SHUN]
And of course, females don't have beard in vanilla.
Just because players have, for a long time, taken advantage of the fact that the game is incapable of making any sort of moral judgement of your actions, and is powerful enough to let you perform absolute atrocities doesn't mean that dwarves wholeheartedly approve of what you are doing if they knew what you were doing. To them, "Unfortunate Accidents" really are accidents, and they are more terrified by the death of a noble than they are of any other sort of dwarf, based upon nobody wanting to migrate to fortresses for a year or so after killing a baron consort.
It's a problem with this community that the community jokes have been retold for so long that people have started to forget that they are jokes, and that it doesn't actually represent Toady's vision of the game.