Before this runs the risk of being a struggle session about arming/disarming the Proletariat, let me specify this is specifically about the belief that 
holds deeply that he, or someone like him, may be the lucky winner of a heroic tale of personal vigilante triumph and save the day and get praised for shooting the bad ( 
) guy with a gun.
Notice how that just about never happens, no matter where the mass shooting happens? Police presence doesn't often matter either, not until the shooting is basically done, unless one of their own gets shot first.
What excuses have you heard?
A common one around here is a classic: "everyone is too scared of getting sued these days," with the implication they'd be waving their guns more regularly otherwise, maybe shooting them as a means of communication. 
A more specific and spicy take I've heard is "because guns aren't allowed in schools so only bad guys bring them." 
(Ask a teacher who has guns in school and they'll tell you, for what it's worth)
spoiler
Bc they're scared in general of getting shot. People have no idea how they're gonna react and that situation and it's pretty common to completely freeze up in shock. This even happens to some people in the military.
That's actually a fair and understandable reason, not an excuse as far as I'm concerned.
By contrast, all the usual tough guy talk from wannabe "good guys with a gun" about what they would supposedly do in that situation is pure
tier internet tough guy bullshit. 