I haven’t taken any psychology courses for well over a decade, but I’ll be happy to explain.
From what I recall, cluster B personalities aren’t genetic perse but rather they are formed from
environment. Sometimes the result of being raised by mentally ill parents.

I believe that trauma, more so from parents that are negligent (emotionally or otherwise), does contribute greatly to a person developing a cluster B personality. I think depression is overdiagnosed in a lot of men who demonstrate those traits, especially if
The National Institute of mental health is to be believed.
There is a recent source on
Incels expressing higher levels of depression than average. If you’re interested in more reading.
Of course, there’s people who become cluster b even within stable households. But I personally believe it has a lot to do with early childhood in the cases of many incels.
Many times, you’ll see these men go from viewing their love interest as well, a love interest, to viewing her as a harpy within mere seconds of rejection. Most normal adults who can regulate their emotions would go “It’s alright if you don’t want to be involved romantically”, and then go for a drink later that day (or do something they enjoy, and get used to the prospect of their crush not being with them romantically).
However, many times incels may either ruminate in their depressive thoughts or start seeing the person they confessed to in the worst light possible. May go as far as to call her a bitch, whore, slut and possibly take it a step further. This isn’t a normal reaction, it’s very much in line with the symptom of “splitting” you’d see in Borderline Personality and Histrionic personality disorder.
Truthfully, my knowledge might be a tad outdated because as I prefaced earlier, it’s been years since I’ve taken any of those courses.