I wouldn’t be looking for much logic here. The filing off of the serial numbers tells me that he thought of killing the ex with the intent of getting away with it. Between the filling off of the numbers and this spree, things obviously changed.
He could have, easily. He was able to roam around unmolested by law enforcement or security from 7:30 until he killed himself almost 8 hours later.
Law enforcement didn't even figure out what his name was until almost 24 hours later.
If his intent was to kill his girlfriend and escape, he could have hopped in a car at 7:30 and taken an international flight out of DC or even Atlanta before they even knew who they were looking for.
At the time he shot himself, he had yet to see or come in contact with a security guard or a policeman.
If he was behind the bomb threats, timing police response, he may have been planning to get away with it all.