Idk, I struggle to see how someone with a poor social network is a threat. I suppose it's possible for them to become unhinged and lash out, but I highly doubt that's what most people interpret an incel as. So I think it has more to do with elimination of discomfort than removing a threat. The same could be said for an elderly homeless person, who causes discomfort by being dirty and smelly, but isn't really a threat.

Then again, perhaps different people have different motives/triggers, people are complex and I'm not a psychologist or anthropologist.