What have I done?

Gollum...well, actually Sméagol, was (as Frodo says) not unlike a hobbit once, and we see that here. He's a very hobbit-like creature. He's a simple country bumpkin. He used to fish with his buddy. We have no indicators that he has a history of violence or anything. The meat of it is Sméagol is a decent creature. He sees the ring and the ring instantly looks into his heart and says "Oh, there's darkness here I can work with. There's greed or there's coveting". And so, he wants it from his friend. He asks for it. The friend doesn't give it to him, so he kills him.

Now, I want you to imagine...You're a decent person and you've done something terrible. Which is true for a lot of us. The sheer amount of guilt and shame and even trauma that comes from believing that you're good and having to square up to this terrible thing that you've done.

So Sméagol runs away to live in a cave of gloom and he's crying out because he's alone, but he's also alone because people cursed him. They spat on him for being a murderer. So he has some trauma going on. There's the trauma of killing his best friend. There's the trauma of being rejected and there's the trauma of living with what he's done. My read on this is that Sméagol wants to believe in his own innocence. And so, his brain created another personality to house all of his darkness and to take the blame for all of it.