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 society? What? To protect those folks who haven't taken the time or had the inclination to learn to protect themselves? I don't think so. I help save some poor schmuck behind the counter and I get sued by the family of the deceased, not the schmuck behind the counter. Why is it my duty to risk my life and livelihood for some guy who has bothered to take care of himself?
I haven't purchased guns, gotten training, and practiced so that I can go play super hero.