In the end nobody but Maria and Gerald cared about Shadow as a person. To then he was a disposable artificial being that didn’t fulfill it’s purpose. Maria was his only light, the only person who gave his life any meaning, and they took her from him. His condemnation and pure loathing for humanity is entirely understandable, to be honest even if Gerald hadn’t altered his memories Shadow would have probably still ended up on the path he did.
But because Gerald altered his memories, it likely saved Shadow from becoming a monster. He could blame that hatred, all of those dark emotions on someone messing with his mind, he didn’t have to face the truth. It wasn’t until his own game when he did, and when confronted with the truth, confronted with the darkness, he chose to again run away. He “abandoned” it, not because he hated it like he said, but because he refused to think about it.
And now every time he does think about it, it hurts more than anything, and now he has to face it again, all of that pain is brought back just to be taken from him again. This time however, he won’t be afraid anymore, he can face it now, no matter how much it hurts, because he isn’t alone anymore, no matter how hard he tries to be, no matter how much he pushes everyone away.