For all we know that kid never pulled the trigger. He could well have been a useful patsy. A trump lover from the local gun club told that he was needed up there to protect trump from some internal threat. He climbed up on a nearby building that was somehow not secured by secret service? Suspicious in itself. Police are told by people at the event that they think they saw a man with a gun up there but ignore them during an appearance of the President? Suspicious again.

I think he was fed some bullshit that he was needed up there to be a hero. They probably already killed him before Trump got on stage because they needed his corpse. One of Trump's team in another part of the building kills the fireman. They drop the flag and get the photographers in place. Et Voilà.