I'm more of the opinion that Kotonoha going off the deep end was catalyzed primarily with her getting
Than being two-times by Makoto. I mean yes the psychological and emotional burden was there because on Makoto, but if she didn't get what came to her perhaps she would never have initiated said boat. I can't even remember if she was already knocked up by that time.
Of course that does not exonerate Makoto being a complete testosterone-driven asshole but you get my drift.