So here's my theory, which is pretty far-fetched, and I won't be surprised if it turns out I'm completely wrong, or if I've forgotten something that happened which contradicts my theory.
I think Tonogai is Kazuya. I think that at some point in the future he'll awaken to the fact that the real source of all his problems has always been him, and get another shot at reliving that time period, but he'll come back as Tonogai instead of himself. That's why Tonogai's in love with Anna and hostile to Kyou, that's why she says she knows "better than anyone else how much of a miserable, bottom-feeding shitheel he really is," and that would add retrospective significance to this bit:
http://dynasty-scans.com/chapters/umareru_kachi_no_nakatta_jibun_ga_anna_no_tame_ni_dekiru_ikutsuka_no_koto_ch11#11
It's obvious, even if I'm wrong, that she's from the future, just like him, because she has the same kind of foreknowledge. If I'm right, then he'll probably figure in the end that his only shot at atonement will be suicide, but then think better of that and look for a way to try and undo the harm he's already done; hence coming back to kill himself ahead of time.
But maybe that's just optimism--thinking there has to be more to him than simply the worst human being ever.
