What happens if some guy who is bigger than you sucker punches you at a bar fight? You're just going to shoot him in the chest?
I guess the sucker punch would be free (except for that little assault charge), but I won't take a second one. There is a big difference between a single, unexpected punch and a committed sustained attack. It all boils down to "ending the threat". If the SOB is content that he "got you" with the sucker punch and has no intent to continue the assault, then the threat is over. Put your ego in your pocket and let the legal system do its job.
As a teacher in a juvenile correctional institution, I was assaulted once. The lad was not happy with his test, stood up, picked up his desk and threw it at me from 20 feet. I saw it coming, was in a swivel chair and praise the Lord he threw straight with no spin and I caught it. He was not in my face and beating on me, so I put the desk down and informed the security staff. It would not have justified my going to the student and applying any type of force. He did pick up a second desk, but by this time staff was there and I easily stepped aside and let it hit the wall (he threw hard and shattered the desk). Staff was able to restrain him at that point.
Had he come over to me and started attacking me directly, I would have been justified in using any force necessary to stop his attack. True this was in a situation where no "weapon" was availible, but if I had to break his scrawny neck to stop him, it would still be justified.
Back to the above situation in the bar. (I don't go in bars.) For someone to get the chance to hit me, it's going to have to be a sucker punch or an attack without warning. One punch can do significant bodily harm and I am not legally required to take an @$$ whippin' just because he didn't mean "to really hurt me". ("I was just funnin' ya!")
If a fella says he's going to kick my @$$ and doesn't stop
, I'm not going to let him have the chance.