You thought the guard went after the guy that got splashed but he went after the guy he hit first. The guy he hit first stumbled off to the right out of frame, that's who the guard went after.
Context is everything...we dont know what happened before or after.
To me it looks like a McDonald's employee asked security to assist him in asking 3 customers to leave.
At some point the security guard recognized an imminent threat to himself and/or others and decided to neutralize the threat without firing any mechanical weapons.
He didnt steal on anyone as he made his intentions well known by adjusting his pants and chambering his chin checkers.
It seemed to me it became personal and not about the job. Either they knew him or they acted like they knew him. Well right now they know him for sure. Cottonmouf was saying the old saying don't start no shit won't be no shit. He showed them just how far to go with him. Leave him alone or suffer the consequences.