My single biggest concern with engaging an active shooter is this- how does a responding LEO discern me from the bad guy? From what it sounds like some here plan on going bad guy hunting if they are in a large building/park/campus etc. What is going to run through your average LEOs mind when he sees you armed and on the move? Put yourself in their shoes. Not only do you run the risk of getting shot by first responders there's also a good chance you make headlines as a second shooter. Not that the latter is critical or even matters at the time but that's not how I'd like to be remembered.
I suppose it all comes down to location and timing. If I don't have to go looking for the shooter then by all means I will do something about it. But if I have no visual and just hear gunshots, I'm keeping my weapon holstered and looking to put distance between myself and that noise. If that's not an option I'll improve my defensive position as best as resources allow, draw my weapon, and be ready.
Sgt. of Marines, 5th Award Expert Rifle, 237/250
1st Award Expert Pistol, 382/400
D Co, 4th CEB, Engineers UP!! OEF 21JUN-20SEP2011