I watched the video just now. The guy is about 300 pounds and slow as cold molasses but he could still have a knife (or the girlfriend could). I still can't say I would physically intervene for that reason and ones already stated although I've probably got a 70-80 pound advantage over the guy. Also, the victim's injuries were not quite as severe as the OP originally believed. In light of all that I would probably have still drawn down on the man.
I consider those who cause innocent people to be defenseless through advocating and enacting "gun control measures" to be murderers by proxy.