Just curious - if taken literally, if he pulls a knife on you and you manage to wrestle it away from him, and he then gets stabbed during the struggle and dies, then have you committed murder? After all, once you have gotten control of the knife, you then have the upper hand, you are armed and the perp is not.
No, in such a case you would not be considered guilty of murder. You are still acting in self defence - getting hold of the weapon was part of you defending yourself and as he was still fighting you, he might have managed to retrieve the weapon so you are still under threat.
Even if you brought the knife into the room - say, you heard the burglar, grabbed a bread knife and went into the room, once he attacks you, if you believed he might seriously harm you, you'd probably be justified in using the knife.
If, on the other hand, you got hold of the knife, knocked him unconscious and then slit his throat, you would likely be guilty of murder. Because by that stage, you are no longer under threat.