But a lot of them act in bad ways because they believe/have been told to believe that their difficulty in finding a relationship is someone else's fault.
This often makes them utterly insufferable to be around, which is a positive feedback loop.
And then, hey, look, here's a disaffected community of angry young men, all feeling that society has taken a great big dump on them, all ripe for radicalization by some cynical ideologue.
Thus, while it may linguistically be the western version of some exotic-sounding Japanese word that translates as "single adult", the implication is the exact opposite causality of your question: they are single because they are bad, not vice-versa.
Being an incel is something you choose. Everyone else just isn't having sex.
It is common to see some one with that mindset to see outcomes as inevitable or as a function of things wholly out of their control such as looks or a perceived social dynamic involving 'chads' and 'staceys' that they are ineluctably locked out of.
Just some real bad vibes.
They are bad people because incel forums are normally filled with crab-bucket mentality users who regurgitate highly misogynistic tropes up until the edge of being openly violent to women.
If you are unable to see the misogyny towards woman in many incel communities for whatever reason, at the very least I would say that they are bad for men because they tend to grow by reinforcing the idea that their community will never be good enough, that there is nothing that they can do, and it is the fault of society for all their problems.
If you are going to sit here and pretend Elliot's manfesto wasn't misogynist, then so be it, there's no point to carry water for these guys.
The first paragraph speaks for itself. The whole "involuntary" part of "incel" implies that your celibacy - the fact that people don't want you as a partner - is totally out of your control. Incels are the epitome of "nice guys".
The implication is that society is also very hostile to men and boys right now, demonstrated by them becoming incels in large numbers.