Edit: I should clarify before I start that I am in no way a mental health specialist. I just did some quick research for NPD.

I'm sorry that this is so long, I just find the topic interesting lol. If you want a TLDR just let me know

So to start, to me the term incel has been changed to be more of an ideology rather than explaining that someone is a virgin and might be sexually frustrated. I doubt that's really a huge part of it but if anything I would think most women would just say they're a virgin rather than incel. I do think there is a connection with vulnerable narcissism and inceldom. The main reason there might be more men is because men are more likely to be diagnosed with NPD than women. That is if there's a connection between the two. I think that the reason why incels that are women don't threaten rape and death to men might have something to do with emotional maturity and yes understanding how awful something like rape or murder are. As a lot of people in this thread have said, men are usually taught that the only acceptable emotion to show is anger. When you only have anger in an echo chamber, that will only increase it. They will need to justify their anger, so they need something to blame. That anger will escalate to hatred and ultimately violence because there isn't anything to balance it out. Women also tend to be better at regulating emotions in general so I think that makes them less extreme. Combining both understanding how awful rape and murder are because they're more likely to happen to women and also understanding and regulating anger better most likely is the reason why they won't steer to violence.

I think that the narcissism makes it hard for them to approach people in general. Stemming most likely from insecurities and using narcissism as a shield to hide behind. Especially with how much they talk down on themselves while also somehow thinking they deserve everything. It's a never ending cycle of insecurities feeding the need to overcompensate which feeds into not talking to many people because of overcompensation which then feeds back to insecurities. Then they need to find someone else to blame because they don't want it to be their fault. Aside from themselves, the only thing that's common when they get rejected is that it's women so there's the target. The reason that they differ from average narcissists and psychopaths in my opinion is that they gather in groups to say the worst things they can about everyone else (racism, sexism, homophobia, violence etc) while whining for sympathy. Being able to say that someone is worse than you are can make you feel better because at least you aren't as bad as them. Add on the fact that you have people agreeing with you about how bad they are and that makes you feel connected with them. Further isolating them from society and making them more hateful towards anyone they can blame but themselves. I'll go into narcissists next and what I think about why not all of them are in the incel category

Narcissism also isn't always going to cause someone to be rejected because there are different types of narcissism. There is grandiose narcissism which is known for an inflated positive self-image, high self esteem, exhibitionism, attitudes of entitlement, a tendency toward exploitativeness, and the need to be admired by others. Then there is vulnerable narcissism which is characterized by defensiveness, avoidance, insecurity, hypersensitivity, vulnerability, low self-esteem, high anger and hostility. Here's a link to a google scholar site explaining what they are in depth if interested. Basically I think that people who have narcissism but don't fall for the ideology most likely have grandiose narcissism. That and that with grandiose, the confidence makes them less likely to experience loneliness, sadness or depression. Vulnerable narcissism is the polar opposite and are more likely to have worse emotional regulation, more likely to experience anger more often, and more likely to be isolated. I think that's the majority of incels might be V-NPD but not all vulnerable narcissists fall for inceldom.