I'm not sure that guy did anything wrong during what is shown of the incident in this clip. Depends on who did what before the clip. It could have been that he did something criminal and they were trying to arrest him or he could be defending himself against a dangerous attack while he was down. Both scenarios are plausible.
He almost lost the rifle to the mob. Pretty obvious that having armed civilians living out their hero complex during a riot, is a pretty stupid fucking idea. He might as well have gotten himself killed and armed a criminal.