I bet he was a bigtime Democrat and wanted revenge on Trump Voters who didn’t vote for Hillary. I bet most in the consert were Trump Voters—Just saying—its a theory.
The idea I see as the most plausible is the money angle - that his financial situation was a house of cards, and was collapsing on him, and he “blamed society”, and in particular Las Vegas, and this was his revenge against Las Vegas and the world. As for killing all those people - it meant nothing to him - it was just the most horrible thing he could think of to do.