I pray every day that I am not faced with that scenario but should that happen I ask for a clear eye and steady hand. I also feel there is no duty to engage, that is why its called SELF defense. We hear all about the split second decisions the police have to make all the time in these situations too but why would that be any different with you and I? We are not blessed with the authority of the state nor the protections of it either. If the police show up at an active shooter scene and he sees you have a gun in your hand, you will die at the hands of the state agent and he will get away with that, and receive an award on top of it. You will just be the "good guy with a gun" that thought it was a good idea to engage an active shooter that falls into obscurity.
I just know this, all situations are different and If I am faced with that some day I will make a decision accordingly, hopefully I will survive it.