Unnecessary sucker punch (they're always unnecessary).
He could have left the scene....just walked away. What's a drunk going to do? Run him down?
It's not like he was at home defending his wife and child. Just loitering at a C-store with some drunk dude.
But he probably felt tough, thought he could pawn off the advance as a threat. Guy is a coward.
Anyone with any sense of how streets work knows this: if you can run away from a fight, you run. You never know if the guy you just suckered has a group of buddies waiting around the corner to "help you". Anyone who's ever bounced at a bar knows you don't randomly punch people unless you want all their friends and the cops coming after you...you defuse the situation, see if you can separate yourself from the other party, and then do it quickly.
The drunk wasn't a threat. This guy is a coward.
Hope the guy gets a lot of time in prison.