I remember that was very annoying, but I am sure that a mob going into deadly state will be knocked back once more, so it is possible to knock a mob away (or down), then knock it back once more when it hits deadly. Also, if dual wielding, you can knock a mob into deadly with your first strike, then instantly finish it with the second strike in mid air. I think it was like that at least... I suddenly feel a little bit uncertain about this is right or not.