You can use deadly force if you are met with deadly force.
Also, you can shoot a person if they force their way in to your home. (In most states, it's the "make my day" law)
In this case had the poor chap who was getting worked had pulled a gun and shot one of the guys, he probably would have been charged with murder, attempted murder, or at the minimum assault with a deadly weapon. (Again, in the USA)
Maybe they were mafia dudes and he fucked with them, but if not then that guy needs a serious gut check and even so he still needs a gut check. I don't give a shit if you are Bruce Fucking Lee, you hit me then you can bet your ass that if I don't get back at you one way then it's going to be another.