How about this article from People. It says Paddock had a no good, bank robbing, psychopathic father, and psycho tends to pass from father to son. The article also says that Paddock also had a very bad childhood.
None of this explains the very real phenomenon that lone gunmen don't have jobs and girlfriends.
Why have 24 guns at the crime scene? Pull up a couch.
The guns were Paddock's babies. They were his trophies, his friends, and an extension of his id. Paddock felt about his guns the way Floyd Mayweather feels about money. Why didn't Paddock travel light and take his best two guns with a thousand rounds? He wanted his whole arsenal to be there with him. Why did he do it? One does not collect 24 assault weapons without thinking about killing people now and again.
I think there is also a thrill motive for Paddock. You don't gamble for 20 hours per day without having a bit of an obsessive thrill seeking personality. Also, gambling probably didn't push his buttons as much as it did at first.
How then does he have a girlfriend?
Beats me. Never heard of such a thing.