I don't think he ran out of ammo, because he went down to the Cafeteria and started shooting at a propane tank. I have no clue why they ran around the school shooting at useless stuff like lockers and into empty classrooms. I think they just wanted to do whatever damage they could to the school. They did hate the place after all.
I think they kept going back down to the Cafeteria to try and get the bombs to explode. When that didn't happen, I think they probably thought "Fuck this" and decided to kill themselves. They knew that police would probably be coming in soon. I mean the place was surrounded by police.
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"I’ll see you in Heaven if you make the list"
Zachary Patrick Bowen (March 7, 1995-November 5, 2021).
I miss you little brother.