Hell, if we're going to speculate, try this:

Maybe the shooter recently found out that he had a fatal disease and would die soon; rather than go quietly he may have decided to go out with a huge splash, i.e "an existential attention whore" who no longer gave a flying fuck about anything anymore.

Or maybe it was an "anti-Trump" political statement; since he cannot get revenge on our leader, he may have targeted a group of those whom he assumed supported Trump, i.e. fans of country music.

Or, he could just be crazy as a shit house rat.