On the radio (Wayne Allen Root) from a supposed friend of his host, said that he recently won a $100k jp. Guy even paid off his marker before his rampage. I'm not sure what they consider recently but even WAR was like "who does that?" to the idea of someone paying off their marker right before their rampage.
To me, that seems a strong sign that his gripe was not with the casinos. If his grudge was about money or casinos, it seems natural he would stiff a big marker in his way out.
Maybe a target of opportunity, the music festival. He played in Vegas a lot, he probably saw a lot of big events come through, perhaps experienced some inconvenience with the crowding, traffic, maybe some rudeness from people in town for things (not just this concert). But they have a lot of evidence he was scouting other music festivals for most of a year. So I'm guessing that was the focus, not gambling.
I doubt we'll ever know for sure. But he liked the whole casino experience AFAICT. Just not the other customers.