The only situation I can evision where I'd take on an active shooter would be if he was right there in front of me shooting down innocent people. I'm no rambo who carries an AR in my trunk hoping to play cops and robbers.
My eyesight is not so bad that I could not see a man firing a rifle into a crowd.
On the otherhand to the shooter I'd be just another person in the crowd of intended victims till I opened up on him.
As for feats of legendary shooting prowess, anyone who can't put all his shots in a human torso at fifty feet or less needs to practice , I've had a lot of practice in the last fifty years or so.
If someone throws down on me they might as well shoot because that may be the only thing that saves their life. Then they can be the one to go to court and explain why they shot the man who brought down a psycho mass murderer before said psycho could claim more victims.
Its not a perfect world, and I'm certainly not perfect. If my blood is up theres no telling what might happen.
If a psycho is shooting people right and left, people will go in all directions, except towards the shooter. In a moment the shooter will have cleared an area around him self one way or another.