There's one thing that I've read, that gives some potential insight into the guy's thinking and motivation. His father was a bank robber, who featured on the FBI's most wanted list, back in 1969. His father was diagnosed as psychopathic.
It strikes me that a son, who may have come to idolise his father, could have a deep rooted desire to outdo his father and become an even bigger criminal, in one massive attack.
I doubt that the music had anything to do with the decision to attack. It was most likely a familiar venue, with a hotel overlooking it. It was probably ripe for attack, as an outdoor concert, with vantage points that could be booked.