A few recent articles with some new information.
The closest they've gotten to explaining the massacre is that Paddock lost a lot of money since 2015 and became depressed because he was so proud of his status as a high roller. It's not really a motive but it does help explain things since like so many other mass killers, he may have started to perceive himself as a failure despite his wealth.
They also said that he looked up police procedures on the internet, probably in preparation for the attack.
[You must be registered and logged in to see this link.]Also a two part interview where some of this information came from. I haven't watched it yet.
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