Wow. Busy topic.
The problem with the police is they are much like EMT's: they show up *after* the carnage. For example, a shots fired call I placed less than a year ago the response time was over 7 minutes. Way too long for the police to do any good for anyone.
To get the phone, call 911, explain to the dispatcher what's happening... the scenario is finished before dispatch sends the call out the cars. Most fights in the real world are over in 40 seconds or less.
As for a good samaritan coming to the rescue, I wouldn't hold my breath. I won't dare to even break up a fistfight in the Wal-Mart parking lot. It isn't that I'm not capable, but I'm not introducing myself right smack in the middle of people fighting about who knows what. I simply refuse to open myself to the legal liability of inflicting bodily harm on someone when myself or my family are not immediately at risk. The legal system would not look kindly upon a highly skilled martial arts instructor beating up on a poor, untrained civilian. I'd need an equivalent of Johnny Cochran to win -- a lot of money out of my pocket to defend someone I don't even know. Ain't happening.
One of my fellow students broke up a stabbing. While the victim was busy trying to hold his guts in, the assailant turns on my friend. The knife guy hit his head on the concrete when he landed and never woke up. The DA refused to press charges citing self-defense and the victim. My friend is still carrying the guilt around with him some six years later. The guy whose life he saved hasn't helped with the psychologist bills.
Guns don't kill people. People kill people. Guns are merely the most convenient and least personal method of accomplishing the task. If I'm set on killing you, guns, knives, axes, baseball bats, broken beer bottles, whatever is around to accomplish the job.