If his accomplice didn’t show, then why continue? Or at least call the valet and have his car brought around, park it over there himself and take a taxi back. He’d have been delayed a few minutes but it wasn’t like the music crowd was going anywhere. They’d have still been there.
My guess is the missing rifle was supposed to light up his car right after the shooting began. So he realized his partner had messed up, escape now not possible, so he continued firing and went down swinging.
Let's say he broke out the windows first. Then he started trying to locate the minivan full of tannerite which let's say was supposed to be parked near the fuel storage tanks. He doesn't see it.
Having broken out the glass, he knows he is on the clock with respect to leading people back to his room. Maybe people see the glass on the ground and look up, tell hotel employees, and ultimately leads to his room.
So he just goes ahead and at least shoots up the crowd.