The way I see it is: I am an armed citizen, I do have a moral duty to my fellow citizens. I cannot sit (or run away) when somebody is committing mayhem if I have the tools, opportunity and the law (Florida Stand Your Ground)behind me to stop the attack. Without doing anything foolish to make me a target, I'd smartly engage the SOB, if nothing more thatn to give everybody else a chance to hide or run away. And no, I am not having a Rambo/Dirty Harry complex but I think it is morally right to do something for your fellow human beings in dire straits. If my family was in a mall without me and some nutjob decided to go in a rampage, I would think some other armed citizen would step to the plate and help save them.
Engage at first / best tactical opportunity - ambush