If sober guy shoots the drunk guy he better have a damn good reason. No different than if it was in a bar, in the street, in a store, in front of a bank, in a bank, at the mall, at the local Target...
I don't know. Maybe he is the DD and trying to keep other people from breaking the law by drinking and driving.
And having a gun is not illegal until you set foot inside those four walls and someone else decides to drink.
Expect for the fact that violence in bars and restaurants has not increased in states which allow carry in them.