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Because a lot of incels relate to him and he was weird as fuck

You're talking about how everyone contradicted Elliot, but you have to remember that the manifesto was in his view. That's how he saw and interpreted his life, and because he was mentally ill, it obviously wasn't reality. He saw his stepmother as evil, he saw people as bullies, his mental illness warped his vision.

Adding on to what others have said, ER almost certainly twisted the truth to paint himself as more blameless than he really was. The manifesto was, despite every admitted shortcoming, his attempt to build his own public image. I imagine the reason why he never mentioned any previous killers in his manifesto was to appear more original than he really was and too keep the attention focused on where he wanted it - himself. Talking about Cho or Sodini would've been too much of a tangent for him.