i always say Cassian is the poster boy for incels, and i've always meant it but now i've actually put together a coherent explanation
incels have this ironic habit of hating women but wanting to prove to themselves and each other they can attain them like prizes, usually just to have sex with them. i fully believe Cassian hates Nesta but wants to prove that he can break her, and when he succeeds he thinks his dick is a good reward. he loves playing the victim, his ego is simultaneously huge and so very fragile and he loves to blame Nesta for his made up oppression. ie. the hallmark of a good incel.
incels believe that everyone has a "sexual market value" (SMV) that depends on their attractiveness and desirability, they believe their own SMV is low and that the romantic "market" is rigged against them. Cassians version of the SMV is the fact that he's a bastard and believes his relationship with women is rigged against him because of that, he resents Nesta for this because he is involuntarily celibate because she continues to reject him. he finds this frustrating and unfair and gets aggressive, just like incels who are "rejected".
incels believe that women are only attracted to wealthy, high status men and feel sorry for themselves because they are rejected. Cassian believes Nesta likes Eris and the men she was groomed to attract as a child and feels sorry for himself to the point that he makes her grooming about him not being good enough.
incels criticize women for "promiscuity" but then complain that they won't have sex with them, they believe women should be chaste but also provide men with sex. Cassian judges Nesta for "sleeping around" but then constantly sexualizes her, believes she should sleep with him, and abuses his power over her to have sex in the house.
incels believe their violence against women is justified, usually to the extent they aren't even aware it's abuse. Cassian continuously abuses Nesta and justifies it because of her attitude, her sex life, her drinking, etc etc, she deserves it because of her own inherent nature and he doesn't even see it for the abuse it is (just like a lot of readers because an abusive incel couldn't possibly be a hot guy like Cassian).
incels reinforce stereotypes of masculinity, gender, and unequal power dynamics. Rhysand and Cassian do this with the sisters and the Illyrian women to uphold their own power but they do it in a way that somehow makes them look like the victims, this is the tricky part about incels who hold power and/or are attractive and the fact that their ideology is imagined because no one sees what they do for what it is
this actually came from someone questioning if Azriel is an incel but personally i wouldn't say he is, he's definitely got his faults but i would say he's more of a yearner if a bit desperate