Maybe because it's been so long since I read them, but I don't recall picking up on really wanting to kill in order to do so.They had a bunch of other spells that could be used to kill in various ways, the only real unique thing with the killing curse is that the one using it has to WANT to kill the target.
So by definition using it means you have no excuse.
You could use the levitation charm to hang a big rock over a doorway and then crush someone's head as they exit, much harder to track.