I saw this the other day. I understand the high level of stress, the adrenalin and the necessity to restrain an outlaw but if the cops can not show enough restraint to prevent them beating an unconscious person then they are in the wrong job.
I know it's wrong but in a way I can imagine why the cops have lost control there. Maybe it wasn't so much a power trip but they probably saw their fellow officer swiped (attempted run over) by this dude. Goes to show we need more emotional control in certain jobs.