If a man points a gun at me regardless of whats in his mind he is a threat that must be dealt with.
Thrill killers often work in pairs.
The case of mass shooters working in pairs or more is a rarity but it happens.
I think its more a matter of some making the assumption that I'm blind deaf and dumb and can't recognize just who in the room is the thrill killer shooting down people right and left all around me.
I am making an assumption that you did not see the shooter suddenly start shooting. You may not have even been there when the shooting started (say you came upon the scene or someone came upon you holding a gun). This is more likely as active shooters move around a bit and people start moving all directions when shooting starts.
If you plan on shooting everyone with a gun (remember you just talked transition fire) as a threat and you shoot them, even the ones who are surrendering than you could just as easily shoot someone who is not the mass shooter.
Anyone following your advice could just as easily shoot you.
ETA- Just as M Leake stated. We posted at the same time.
PID is your friend.