Gambling big allowed him to enjoy the "high-roller" perks but when you show up to Vegas every Friday with a million bucks and leave every Monday with $995,000, that's called "losing" and eventually you're going to run out of money. Paddock was doing this every night.
When Vegas changed the algorithms of their slot machines to pay out a little less, Paddock viewed this as cheating and with his money running dry, he wanted to teach the casinos a lesson.
I've never bought the crazy conspiracies. The simplest explanation is probably the best; Paddock thought he found the secret to living the good life in Vegas on their dime in perpetuity and when he say things weren't going to work out this way, he became angrier, drunker, and blamed the casinos.
It could also be called "money laundering".
I have bought several of them. They are inexpensive and available in an attractive boxed set at a discount. Some assembly is required and batteries are not included.
The "one most likely to succeed" is that Paddock was an Agency informant who turned on his handlers, with a little help from some people the Agency thought he was setting up for a sting.
The "disgruntled gambler" narrative is the Agency cover story to protect the fools who sponsored this project and had it blow up in their faces.
But that is just another theory put together from open sources with suitable cross-checking. There are people who know exactly what happened. They are not going to confirm any of the stories because the publicizing the real thing would make a bunch of people look very bad.