“Perhaps the girl at the perfume counter dissed him. He looks kind of spaced out walking in, like on a mission. Too bad he did not wait around for suicide by cop.”
I think we are looking for clues and ignoring the obvious: Perhaps he is a radicalized jihadi and used the method he would be familiar with. He walks in shoots as many rounds randomly as he had available, put the gun down and fled before anyone could get the cops there, ran off by a pre-planned escape route before any police response could arrive. Terror spread, attack successful and he disappears to fight again. He will be a hero in his world.
It is atypical for a Mexican to murder 4 women. This sounds more like a Muslim. They murder men, women, and children.