I think if you are physically and mentally capable, it is at least a civic obligation to intervene. For me, it is a peronal obligation.
For me, it is part of what being a man is. And God, expects us to stop ruthless savages from hurting innocent people.
It is part of why I carry a gun, part of why I have gone thru several Gunsite classes, part of why I have practiced with thousands of rounds of handgun and pistol ammo, part of why I mentally stay alert of my surroundings. Not so I can go out and shoot someone, but so I can defend myself, my family and help another innocent person from trash and scum that live everyday with no regard toward decent people.
You don't have to intervene like most people on TFL recommend. You can be like the guy who watched that fat off duty cop beat that female bartender and did nothing. You could be like the countless other millions of people who watch someone get beat, mugged, robbed or raped and do nothing. For me, I would not be able to live with myself if I sheepishly crawled away like a coward while someone's mother or grandfather was being threatened with death and I was right there with the equipment and the training to help and did not. I wouldn't want one of my 5 kids to see me on a security camera cower away on my belly.
IF this is your course of action, and you can live with it:
"Well I called the cops and yelled at the bad guy, but he still killed that innocnet person right in front of me".
or
"I tried to punch him so I wouldn't have to shoot him first, but I didn't know he had a gun and would pull it so quick and start shooting people"
or
"I spent so much time deciding if I was safe and could avoid legal problems, that someone died and the bad guy got away to do it again"
or
"I didn't think he would really jam that screwdriver thru that young female clerk's throat, and I told him not to".
or
"someone died and my gun never left the holster, because I was indecisive".
If you can live with your handling it that way, go for it. Enjoy your safe life. The criminals are counting on you.
To each his own according to his own, but, I would really look at how the experts recommend you handle it, and combine that with what you can handle.