This is simply false in most states. In short, you can shoot someone dead if you "reasonably believes such force is necessary to prevent imminent death or bodily harm to himself / herself or another". This does in fact include a guy punching you or trying to punch you.

This whole comment shows that you really don’t understand physical violence/haven’t been involved in it. One punch can easily cause physical damage that can ruin lives. One hard shove can crack someone’s head on a concrete curb. These are all things I’ve seen happen. The idea that you could be “beaten half to death” and then have the presence of mind to decide that it’s ok to defend yourself with a weapon is equally absurd.