Not sure on this one. If one has disarmed an individual, have they not eliminated the threat? I can see a DA maybe wanting to push hard to show that the threat was no longer reason enough to use deadly force.
Having to wrestle the gun away is proof enough that the threat is not over. If, at your command, the assailant laid the gun down or handed it to you and spread-eagled on the floor, maybe then, but there is no way to know it is not a ruse - or has an accomplice - waiting for you to drop your guard enough to spring into action. I wouldn't take the risk. Just don't walk away then come back and off the perp.
Woody