For now you can only kill one animal per pasture/coop. While it is more or less OK with, say, killing cows, when you try to free the coop or a ram pasture, you literally cannot do that. Animals respawn on a too quick pace to do that.
Also, when you suddenly need a lot of meat, it is a real b**ch to click a hundred times on 'kill one animal' button. Making the slaughtering system the same as with relocating animals (one, half, all) would improve farming a lot.