Besides here, I am the only one that seems to question how one shoots oneself with a longarm. Yes, could be sawed off, but usually they jump on that detail. Still difficult.
As far as who would’ve shot him, I seriously doubt a regular citizen. I know this place, and not only are firearms prohibited, but few normal people going there would even dare think of touching a firearm, much less having one and carrying. Even barring that, few regular people would keep quiet about their heroism in such a situation.
Have you ever seen the film "Full Metal Jacket"? 
PYLE turns away from JOKER and stares into space, a strangely peaceful look transforming his face. He places the muzzle of the rifle in his mouth. JOKER No!!! BANG! PYLE pulls the trigger and blows the back of his head over the white tiled wall behind him. SCENE FADES TO BLACK