I observed a man beat up at a bar last night. should I have reported it?

Well, it wasn't in a bar it was outside in the parking lot. He was cussing out someone for parking too close to his vehicle. Then he hit the guy in the face but the man who got hit had some very mean friends. They were playing horse shoes and upon observing their buddy getting hit they ran over and beat the guy half to death. ( He is still in the hospital) The group is the bikers called the Vigilantes and for the most part they keep to themselves and are nice people. I believe this man brought it on himself and should have been beat. When the police arrived I said "I didn't see anything. Did I do right?