Not only can one punch be fatal,
It can there is no doubt. One punch can knock you out. One punch can feel like a fly bite. You never know unless it happens. However, in most states responding to threats or simple assault with a gun will get you locked up.
In NC try shooting someone because they busted your nose. They will lock you under the jail.
Here's my credo: There are no good guns, There are no bad guns. A gun in the hands of a bad man is a bad thing. Any gun in the hands of a good man is no threat to anyone, except bad people.