You sleeping with him is not enough. Instead, he wants to be sure that he has you completely and that means that he wants to make sure he is inside your heart, soul and head. And if you really want to hurt him, you can never give him this realization. A narcissist can never be sure of your love for him to the point where he starts taking you for granted. At first, he will make big, romantic gestures and he will have no trouble whatsoever verbalizing his emotions to you. He is doing this only to lead you on and for you to think how special he is. He wants to make sure that he has left a permanent mark on your life and that is something you can never let him think. Instead, make him question your emotions and your intentions.
Don’t be too cold, because that will make him go away before you get your revenge. But make him wonder if you ever loved him or if all of your relationship was an act. Make him wonder if there is someone else in your life and whether he played you or if he was the one who got played.