A fellow with severe mental issues choking on self-cultivated racial and religious rage would be my first guess, but who knows? In any case, I'd love to hear someone from Georgia tell me about the rules of engagement there. In my state you can intervene with a firearm if you fear grave physical injury to the person getting beaten.
I once had reason to obtain legal advice from a local attorney regarding this issue in Georgia.
I was told to keep repeating,"I was in fear for my/their life/lives", and say nothing else.
IMO...Given the fact the perp in this case was obviously not going to stop being violent until forced to, one probably could've gotten away with using deadly force in this instance.