Seeing as a degree of cosplay is a given at premier showings if this was meant to be a sociopathic metacomment on the film series he would wear a Scarecrow mask or a Bane mask, not a gas mask.
Indeed he would probably have gotten more instruments of destruction into the theater if he was dressed that way as people would assume they were part of his 'costume'.
People who have a tendency to do this sort of thing will do this sort of thing.
Doublethinking culture and events like film premiers too deeply in order to prevent these sort of events never works.
The fact that we haven't had an atrocity of this kind at a 501st get together or a Zombie walk is proof to me that these sorts of people are rare and garner uneven publicity because it serves people who want to limit cultural expression and civil liberties.
Insane killers are insane killers they aren't made insane killers because of comic books, video games, music or films.