He spent the entire book fixated on the ability to bend time, and that time was not what we perceive it to be, and that maybe it can be navigated in a non-linear way. So he decided to do that - he went back in time and made it all different. In his head, of course - only one page before he admitted for the first time that he was bonkers. I thought the ending (I literally finished the book 5 minutes ago) was very sweet, and maybe if that had happened, it could all have been different. But it wouldn't have changed anything, he was way too damaged, he probably murdered the girl in Leeds during the Sutcliffe period, and at least one more that was alluded to in the course of the book. But back on track, the ending was a wish fulfillment thing for him to bend time, rewrite history and make it all OK, in a Disney kind of way. Sad and sweet.