I'm so sorry for your loss. I really appreciate your open-mindedness, though I would totally understand if you're (also) feeling frustrated or even angry with this forum.
I didn't have the chance to meet your brother, but after reading his posts I really wish I had, as we have a lot in common. I think it has been really tough for him to deal with (probably) autism and OCD in his own life while at the same time holding a philosophical pessimistic outlook on life in general. I read that he was also vegan which, from my own experience, makes it (almost) impossible to have a positive view of humanity and life itself, because wild animals suffer immensely as well.
Many of his posts are driven by frustration, but it's clear to me that he was a clever and compassionate guy. You can only be frustrated with something if you care about it. Reading between the lines in his posts, I'd bet that he really loved you and his close family. He was an anti-natalist, so rationally he didn't feel any gratefulness for his birth, but it's obvious to me that he deeply cared about your mom.
I hope you can find some closure and maybe understand him and his decision a bit better through his posts. Please feel free to (privately) ask any questions.