Arms dealing does fit the scene. But the deal gone bad doesn’t quite lead to “let’s kill all the concert goers at the last song of the last day of a three day concert.” Not making that leap.
I don’t see arms dealing. If he was running guns, he would not have needed to buy ARs through gun shops, he would have had his own source. Throw in the detail that he wired 100K to his girlfriend in the Philippenes, ostensibly to buy a house, and sex trafficking seems more likely. He had a hidden source of income and the gambling was his means to launder it.