I posted an article a few days ago where the author joined an online group of incels. According to the article, they typically refer to women in general as "femoids," or "female humanoids." That's *all* women, not just single ones. The old-fashioned word "spinster" kind of describes a woman who isn't married or dating. It has a negative connotation but not to the same degree as "incel" does.
I guess the reason that "Incel" is seen as such a negative is that being unhappily single often comes along with violence, threats of violence, misogyny, and resentment. In contrast, unhappily-single women are typically more depressed, bitter, and crabby but not violent. Except maybe in women's prisons.
Re: the term "femoids", the femcels have a similar term for all men, possibly "moids" I think I read.
But as I addressed in my above post, men have a much higher tendency towards aggression and violence across the board, including and especially towards each other, not just towards women.
So I don't think that can be an attributable characteristic of incels per se.
Men just have a more violent nature, either biological or sociological or both.