IMO, he went to party to check out site and/or to plant bombs. He was reported as nervous and irritable at the party. After he checked out the site and/or planted bombs, he went home, picked up his wife and proceeded with his plan. IMO, this was planned. Dropped off child with mother in law, had the pipe bombs and other incendiary devices ready, go-cameras ready to go, and AK's and bullets.
I think I read somewhere that the shooters reloaded after initial shootings, while the victims were shaking in fear. This sounds like cold-blooded terrorist act.