How threatened I feel if I'm alone, and how threatened I might feel in the car with a wife and 2 year old are two different things.
Remember...the guy stopped his Range Rover after the rider did his cute little brake-check number - which, if you watch the video, is pretty obviously intentional considering he was in the RR's lane prior to it.
Would this be a different discussion if they had been assaulting his car with his family inside and then he shot one of them?
Don't provoke the animals. The guy he hit was certainly pulling a dick move, but the mob seemed fairly calm at that point. Even if the crowd was pounding on his car, he's in a giant sealed metal box. From what I saw, the situation at that point was far from dire. Things went pear-shaped when he decided to assault multiple people with his vehicle. People who were just stopped in the road. They weren't being aggressive, just blocking traffic. This is why we have the police.
Now, if immediately after hitting Mr. Brakecheck the bikers had started tearing the doors off the hinges, then yes he'd be far more justified in doing whatever he needed to do to protect his family. But from what I saw, the only time the bikers actually became violent was at the end, after he had run over several bikes and riders, left the scene of two accidents (looks like he knocked over another bike leaving the first stop down the highway), and then ran out of road. Both sides clearly share heaps of blame in the incident, but I'd really have to pin it on the driver for escalating it.
I ran into an incident like this myself, a few years back. Some guy in a big truck tried to cut me off in my car by pulling out of a driveway between me and the car in front of me. I didn't make room for the king, so he stopped his truck in the roadway blocking my path, got out, and started banging on my car while cussing me out. What did I do? Just looked at him. Looked at him and pointed at his truck, the one he needed to get back in. After about 30 seconds, he huffed and puffed himself out, then waddled back to his truck and got in to leave. That's when I noticed his wide-eyed kid sitting in the passenger seat. Couldn't have been older than 10. Made me extra glad I was the one keeping the cooler head, because no kid needs to see his parent going to jail or the hospital. What's wrong with people!?