This is the long-term result of cheating. Setsuko has been traumatized by Yukino, and this chapter shows that she will never be the same or trust Yukino ever again. The slap was needed, as Yukino was obviously just going to give a weak "I'm sorry," and expect Setsuko to let it go. When they properly communicated, it was good but bitter. Her using alcohol to run from reality is insanely dangerous, and who knows what could've happened. I guess Yukino, the serious one that is anxious and afraid constantly, is the one that wasn't taking the relationship seriously.