I feel like most people use it as a cheap insult. It's basically just calling someone a loser.
Losers are always going to be ostracised. However, it really riles the normies up when they commit an act of violence. Then they can point to them and say "see!!!! these people are EVIL and VIOLENT so its RIGHT to bully them!!!!"
Look at the otaku panic in Japan. As a result of the crimes of Tsutomu Miyazaki, people demonised an entire subculture (otaku). Truth be told, these people were already seen as degenerate losers, but now they had ammunition, they could point to Miyazaki and say "see??? these otaku are violent, child murdering pedophiles!!!" This later also happened with another murderer, Kaoru Kobayashi, who also caused otaku to be demonised and considered as potential sex offenders despite not even being one whatsoever.
To me, it's the same shit, all over again. The Satanic panic, the otaku panic, the goth panic that came after Columbine, the new school shooter panic, the incel panic etc. it's all the same shit.
Losers and loners will never be accepted, and normal people will always take advantage of one loser snapping to paint all of them as violent.
To me, people who identify as incels are often misogynistic to an extent, often as a result of real-world treatment (not that it makes it right). However, they are often self-destructive and suicidal, rather than homicidal.