This sounds similar to a question that I posed in a thread I started here a while back.
Here is that thread. Someone mentioned using the weapon against the attacker. I replied that as tempting as it'd be, it may not be a good idea. Someone else chimed in by saying that it would be ok to use it.
If we were struggling over the weapon, and it went off and he was shot with his own gun or stabbed with his own knife, if I took it away and used it against him...as it was said in the other thread, a threat is still present, and depending on your actions after, you could be justified. Of course, I also posted
this comment in that thread. That comment was made by a LEO, so I do place value in that. If we look at that post, and then put ourselves into the question you asked, it seems that some of the factors of you using the weapon are gone. This is where my thinking was going, in that other thread I linked, because I compared this to an empty hand situation, where the threat is over,
ie: bad guy is on the ground, no longer attacking, and you haul off and kick his ribs 8 more times for good measure. Was that necessary?
Same thing here...you get the knife away, and now you chase after him with it, cutting him.
Like I always say, assess the situaiton presented to you at the time. And my old saying, "I'd rather be judged by 12 than carried by 6."
Could the BG retake the weapon after you've disarmed him? Anythings possible and we could what if this to death. What would I do? Gaining control of it is the #1 priority IMO. Perhaps I could still use it against him, but not in the fashion it was intended for. In other words...you disarm the gun and hit him with it. You 'used' it against him, but didn't shoot him.
Depending on the situation, I may use it or may try to remove it from the situation, ie: toss it out of reach. If he goes to get it, hopefully that'll buy me time to get the hell out of there.