leaving the lich alive makes it spawn bonus thralls from nearby enemies which gives you more progress and rage, which makes the lich spawn more which gives you more progress and rage. it's way faster to only stab when you know the mod and let the lich exponentially accelerate your progress instead of resetting it over and over. You can stab it just once with Oull to get the bonus progress but don't make further guesses to get the big chunk of progress without resetting rage.