He got rich from real estate, I believe.
Here's what I think: He got rich from real estate and with his riches became addicted to video poker. He'd travel to Vegas, bet beg to earn the high roller perks, occasionally win big but occasionally lose big as well.
Over time, he was probably winning 97-98% of what he played but eventually those 2% losses started adding up.
>It sounds like the tech changed a circuit board in Paddock's favorite machine when he wasn't looking.<
This is quite possible. The article states tat they started changing the machines to increase the advantage for the house. For someone as addicted to "the crack cocaine of gambling" as Paddock was, this would've been tantamount cheating!
I think this is what pushed him over the edge. It wasn't ISIS or AntiFa, money laundering or gun running. He just started losing more than he could afford to, blamed the casinos and plotted his revenge.