Why did he point the gun at two people not in the scene?
I think they are lying.
I think Alex carried a gun, after the crew walked off the set, he became angry and enraged and add to that he was drunk and high on Coke, waved the gun around in anger and shot two people.
He probably pointed it at the camera so it appears to be pointing at the film audience. The victims were likely right behind the camera. They may have stayed in the line of fire in order to do a camera effect.
Good point