I'm not sure that you can safely label serial killers as incels per se. BTK was married, IIRC. Ted Bundy def. dated. John Gacy was a pedophile (more accurately a hebephile, but close enough).
Many of them were misogynistic for sure, although that's not necessarily a motive when you have serial killers that are gay or pedophiles (Gacy, Wayne Williams--believed to have been the Atlanta Child Murderer--and Dahmer). Incels seem to be much more likely to be spree or mass murderers; the idea of an incel where all women collectively share the blame seems to be a fairly new idea. And a lot of mass murderers aren't related to sex a all, like the people that have been trying to start racial holy wars by murdering non-white people.