Rather carefully planned.
'He had been stockpiling his firearms since 1982'
'"From October 2016 to September 28, 2017, he purchased 33 firearms, majority of them rifles," Snyder said. "We wouldn't get notified of the purchases of the rifles, we would only get notified if there was a multiple sale, which would be two or more handguns in an individual purchase," Snyder said.'
Doing the math he had about fifteen guns already a year ago. He had bought them one at a time to avoid getting flagged.
'Paddock rigged 12 semi-automatic rifles with bump-fire stocks, or "bump stocks," devices which allowed the guns to fire continuously -- much like a automatic weapon.'
'"The ammo clip capacities ranged from 60 rounds to 100 rounds."'
Compare this to 30 rounds on military assault rifle magazine.
From other articles:
-He had installed video surveillance to the corridor outside his suite so he knew to take his own life before police stormed in.
-He possibly visited the site already a month ago during another concert, but it is not known if it was a scouting trip or did he just decide not to go through with his plan at that point.
-He brought in the weapons in 10+ suitcases.
-He had several homes in different states. It's thus possible his girlfriend genuinely didn't know about him having 50 guns.