Agreed but maybe he wasn't planning on dying when he bought the guns and filed the serial numbers. Maybe he first planned on killing the girlfriend and getting away with it then changed his plans sometime between the purchase of the two guns and the execution of the crimes.
I would add that it seems unlikely that he initially intended only to kill his girlfriend and then esacape.
He brought two weapons and enough ammunition to do exactly what he wound up doing - methodically killing dozens of people.
If he intended to pull a hit on his girlfriend and escape, he was not packing light for such a surgical mission.
Nor did he appear to have a vehicle handy to escape in.
I doubt he ever intended to survive the day.