But really, Elliot's biggest problem was that he gave up on things too easily. He even quit World of Warcraft because he said he heard players talking about their sex lives, real or imaginary.
He was basically the prototypical trust-fund baby. He avoided work and effort of all sorts and had a very entitled attitude. He really expected the universe just to provide him with some concubine, with this only contribution to the process having a BMW and some nice clothes (that he didn't pay for).
For a glance into his insanity, when he got a bit older he realized in order to have any kind of money, he'd need to get a job. He did some half-hearted fiction writing, which didn't go anywhere. After that, he decided that winning the lottery was the answer. He'd make these huge road trips to acquire lottery tickets when the pot was really large. Of course, he never won a dollar, and even gave up on that after some time.
He definitely had delusions of grandeur regarding himself and his own abilities. He felt like an unappreciated Chad/genius, even though he basically had nothing going for himself. If he hadn't have had his "Day of Retribution" I'd say there would be a 99.99% chance he'd be a NEET through his 20s.
One of his challenges with leaving his house is any sight of a woman, or even worse, a happy couple would infuriate him. This is to the point where he'd be walking around with some coffee and he would throw the coffee at women, just for existing.
I will partially take back one thing about him: his manifesto is pretty detailed, and very, very long, so he did put effort into something. I actually wish more of these insane folks would take the time to write as large of a journal as he did, because it gives a lot of insight to us after the fact. If he hadn't have written this journal, I'd imagine he would have been an even smaller footnote in history because it would have just been another random shooting spree in CA.