Slinking away and "being a good witness" when you have the ability, the means, and the training to put an immediate end to an imminent deadly threat against the life of another means that you are shirking your solemn duty as a citizen of our society.
Solemn duty to WHAT society? If you are talking NYC, it's not even a society, its a zoo with criminal management and not enough keepers. If you are talking NY, NJ, parts of CA, perhaps MA and many other States whose attitides and practices I'm not aware of, you are talking about a society which DOES NOT WANT YOUR HELP. They think they know how to handle it, and you would be well advised to keep your nose out of it. Certain individuals might want your help now that they are in the situation (and many would not, and many more would sue your buttocks off if it didn't turn out just right), but they didn't want it bad enough to carry guns themselves, or to situate themselves in States that have a slightly more traditional American attitude. As for them, I say **** 'em. And the "society" they rode in on.
As for the dwindling number of States that are not risking the fate of Sodom and Gomorrah, if I were located there, I would certainly do my duty, which is to definitively incapacitate without warning any armed robber holding a gun, IFF it were clear that I could do so safely. That's a big iff, and sometimes beyond being evaluated by someone close enough to terminate the activities of the BG who is showing. Did I mention backup in my first post? Think about it.