Oh, I knew that Kosaka was going to turn out to be Rider, but I thought it would turn out to be a version of him from another timeline or something. Rider's literally him, though, huh? Not a different version; just from a different point in his chronology. So I guess he's now going to have to do everything we already know happened, right?
I guess that Rider's brief monologues about "giving up" or whatever were meant to be about him trying to change things somehow, right? Though if he was trying to prevent this all along, why'd he even keep killing Nagisa in the first place? Is there some force compelling him to? Maybe Nagisa dies regardless of whether he kills her or not, so he's doing it himself with the intent of minimizing her suffering or something?