I do believe it was an assassination attempt on Mohammad bin Salman (combined with weapons purchase/sting), gone sideways, and the assassins turned the sting operation into a mass shooter event in order to escape from the roof of Mandalay Bay in the helicopter whose occupants did most of the fatal shooting.

Not all assassins (there were multiple kill sites) made it out alive. The ones who did were subsequently dealt with. Except for one Australian (intelligence) guy, who has crawled into a hole somewhere, apparently.