Assuming abuse

A man is seen yelling at his son in a parking lot. Dude came back out from dropping something off at a Fedex office and was in there for a few minuets without incident, came back started yelling at his son and his sons friend who was in the car, drags his son out and starts spanking his butt. The kid is 11. Passersbys saw it and decide to attack the guy and his car, while the son tries to pull some of them off his dad. These people assumed the man was crazy and abusing his kid obviously, but luckily he had proof of his actions - His son was prank calling 911 while his friend filmed it on his phone. The cops showed up and the friend kid handed over the phone and told them they were trying to piss off one particular dispatcher, because I guess they've done this before and the dad was pissed. The dad was out cold on the ground and the car is beat to shit. Dad was taken to the hospital and kids are probably in trouble to say the least, but no action against the people who attached him that I'm aware of.

If you came on a situation where you say someone pissed and yelling then spank their kids (on the ass) how would you react?