The only instance that I would consider doing anything similar to what this man did would be to try to save the man who had been shot. In most all states one justifiable reason for using deadly force is to prevent harm from another innocent person. I can't say for sure what I would have done, but I generally am in the "help whenever I can" crowd. If that meant that I had to draw my weapon on these two to render aid to the injured man, then that is what I would have done.
If they had left the scene, or at least started to, then I would probably wait for them to be gone before going to render aid.
I remember such a case .A woman is raped but offers no resistance perhaps because of the shock. The perp then leaves and she follows , picks up a brick and beaks his head severely. She is found guilty of assault !
This could be seen differently than if a man had just been shot and his neighbor was rushing in to protect him, however. Even if the perps were walking the other direction, they could easily turn around to finish him off if they want to because he is helpless to do anything else. That woman had the opportunity to walk the other direction to help herself, she chose to continue the attack.