I feel like I could have been an incel if I had a slightly different personality - more angry, less empathetic, etc. Years back I bought "The Game" with the hope that it might help me - it's the most cringeworthy book ever written, though, so I dumped it after a few chapters. And it didn't help me, obviously.
Also I empathise with the weird because I'm obviously weird, people always say I'm weird, but I've got enough of whatever it is that makes me acceptable and likeable rather than repulsive. And you meet those people sometimes, don't you, who are weird and NOT likeable, just weird, people you'd avoid if you saw them in the street, people you'd never invite to a party, as much as you feel sorry for them.
Some of these incels are probably like that - they know they're weird, people (probably not just women) dislike them or don't connect with them, and the self-consciousness of this makes them weirder and harder to get along with.
Breivik was like that, apparently. A weird guy who wasn't likeable, didn't have any friends, just stayed in his room playing world of warcraft and ingesting far-right mythology - even the other nazis didn't like him.