Sometimes people associate pain with 'other people'. So they want to hurt 'other people' in return as punishment, vengeance, retribution for the pain they've received.
They'll often associate the pain with a particular place, and when the break occurs, they'll go to that place and shoot/attack anyone who is in that place, regardless of whether those individuals are even the ones who caused the pain in the first place.
This guy had an association with the casino (prior visitor to it), which probably is the reason for the choice of place to do this. I don't think it had anything to do with the people at the music festival, I think it was the casino that drew him there. The people were just in the wrong place at the wrong time, but I don't see any evidence yet that he considered them all to be a certain type of people that he hated, it was just that there was a massive crowd of people at this place that had some meaning to him.
It is odd if it was a gambling-related grievance that he wouldn't go down to the gambling floor and shoot there, why shoot outside the window? That is part of the odd thing about it, in my mind. I would theorise that 1) it's harder to get down to the gambling floor with those guns. 2) he had turned his anger on people in general, so thousands of people outside the window enjoying themselves would be like a red flag to a bull.
Obviously there could be something that gives more insight into his reasoning that comes out over the next few days, but I think it's quite likely that we'll never know. As a generalization, men don't usually (compared to women) talk about what's going on inside their head. So unless the investigators can find something in his financial records, a large gambling loss or something, I don't think we're going to ever really know what was happening inside his head.
I don't see anything in the location that suggests anything political or religious, all I see is a hatred of humanity in general, and I think things like that are often a case of you end up with a straw breaking the camel's back, so the final straw might be relatively minor but it comes on the back of a lot of other incidents that have been building up in a person's head like steam in a pressure cooker.
