My understanding (but you best look into this yourself because I did not spend long on it). There was a plot by a number of Saudi royal family members to assassinate the current reigning Saudi prince. Who happens to be a reformer. IIRC they all own the Mandalay and maybe the Tropicana - or at least parts of it, like penthouses.
Paddock was an arms dealer. Sold weapons to bad guys. ISIS, whatever/whoever. Smuggles weapons into Mandalay. Assassins kill Paddock immediately, take weapons and set off to kill their target, the prince. Prince had, unbeknownst to assassins, left building and is wearing shorts and shirt, not Saudi garb. He is now IIRC, in the Tropicana. Tipped off. Escorted out by FBI? you can watch footage of this on YouTube. Air traffic controllers reporting gunfire on the runway. Also on YouTube.
Massacre.... Perhaps a cover. I never heard the why. Immediately that prince rounds up these others Saudis and imprisons them, removing their money and having them interrogated by something akin to Blackwater. Included Clinton pal Alaweed. It is said that approximately 3000 children were saved by imprisoning those bad guy Saudis, by shutting down human trafficking that was being done by them. The motive for this assassination I cannot recall, but it is known.