I watched the video and there was no doubt in my mind that this "plainclothes" policeman was totally out of line.  And since he was not defending himself, he decided to take the law into his own hands, to physically punish these suspects.  He crossed the line and personally he has to pay.  As for the officers who watched...your job was to protect the suspects from abuse even if it was by a fellow officer.   Those who sleep with dogs, get fleas!"