Derp's Favorite

In my opinion, they had some kind of a relationship, either friends with a slightly sexual relationship or boyfriends that never confessed to each other. This caused a misunderstanding between the two of them as one of them thought they were dating and the other one that they were just friends. The one who thought that they were just friends was the kidnapper and the one who thought they were dating/sex friends is the uke.

The uke proably did it with the kidnapper (forced himself on him) in the warehouse as a teenager since we can see that he is all over the other guy in the flash backs. Then they stopped being friends and the uke felt so bad about it and regret it so much that he erased it from his memory. The kidnapper was depressed and felt betrayed so he ripped out multiple pages out of the book where he wrote of the uke as his friend. During the years this memory stayed in his head which made him change from a sweet, shy, cute boy into a cold, cruel, sociopath. The uke remember about this event and hated the uke for it but inside his heart he was actually in love with him and didn't realise it. The fact that he remembered how the wearhouse looked shows us that this memory is forever stuck in his head since this is where the rape happened.

This is just my little theory. I haven't read ahead so im not sure. Im unsure about the details but I'm sure about the rape thing and the staying in his memory thing for the kidnapper.