Best to assume it was a false-flag op until there's strong specific evidence against it being an op.
Hence, Paddock was a particularly loyal/gullible stormtrooper who would fire on innocent civilians when told to do so.
But why was he told to do so?
One day, deep in the bowels of some government office or another, there was a brainstorming session. Mike (do you mind if I call him Mike?) Mike suggested gunning down a bunch of random Vegas folk to accomplish whatever batshit Impact nonsense they were trying to do. Debra (from accounting) said it was a stupid idea.
Counterargument: Debbie's a bitch. Mike himself didn't like the plan that much, but fuck her sideways with a rake, right guys? Therefore, Mike shoved the daydream through planning, through logistics, through strategizing, and so on through everything until Paddock was handed a bunch of guns and told where to point them.
Of course, when it finally happened and they saw the reaction, they didn't even need to say anything. It was obviously a stupid plan, everyone saw it was a stupid plan, everyone knew everyone else knew it was stupid. It accomplished nothing. Which is even better for Mike. Debra is such weaksauce she can't even get folk to agree a plan is stupid when it is in fact stupid. What a loser!
Sure hundred of innocents were injured, but the important part is everyone in the office knows Mike rules and Debra drools. For this critical task, is there any price that's not worth paying?
When you see the government do something that's completely baffling, it's often the case that even the government section in question is aware it makes no sense. That's like half the point.