In this day and age where it seems like everyone age 11 and up has a cell phone, it's quite likely that 911 will be swamped with phone calls, responding officers will get information saying there are multiple shooters as bystanders see you present your weapon, so now the police likely have your description attached to that of the real shooter. Other similarly minded armed citizens are likely to see you with your weapon, assume you are the shooter and shoot you. I know that most people would like to be able to respond and end an active shooter situation if they could. But think for a minute about the reality of what you are dealing with. You are heading in a general direction of the sound of a gunshot with no description of what the shooter looks like, no idea of what he's armed with and no clear idea of where he's at. You know that the police and other first responders are enroute and that they may not have any more information then you do, or the information they have about the shooter now fits your description. Other armed citizens may also be looking to end the situation and may mistake you for the shooter or you may mistake them for the shooter.