Once the movie starts getting well along its way, Arthur becomes only a Microcosm within a Macrocosmic movement that he accidentally started. His work and leadership are no longer needed by the end of the movie, as he shows the way for everyone else like him, and empowers them to lead themselves.

'The Mob' becomes just as essential to the climax of the movie, as his own actions, and without them, he'd never have made it as far as he did.

It's not one guy, one shooter, one disgruntled individual that you need to be concerned about, it's the MILLIONS AND MILLIONS of them out there who are going to say "Fuck this"

Also, Joker doesn't kill people in any special ways, nor does the movie really promote 'Gun Violence'. A knife, a screwdriver, a pair of scissors, your granddaddies' old revolver, anything that does the job of spilling the blood of people that disgruntled individuals view to be the enemy.

The guy is not tooled up with an arsenal of weaponry and a stockpile of money. He's POOR, he goes to a mental health clinic (That ends up getting shut down by the Government because they didn't want to finance it anymore) He's on 7 different psychiatric 'medications', and had already been 5150'd in a Psyche Hospital by the time you begin to get to know him as a person.

You're not going to feel the actual impacts of this movie for a while, it's the kind of thing that causes latent and protracted chaos, not immediate.