Satou’s teacher sees her with Shouko and doesn’t like the fact that she seems to be sharing “dirty little secret.” Of course, Sensei is operating under the assumption that Satou’s parents died early and she was brought up by her aunt in an environment devoid of the love humans need to grow up to become “normal.”
He believes Satou snapped one day, murdered her aunt, chopped her into pieces, and gave the bags to him to incinerate. It’s as good a theory as any judging from the evidence he has…but he doesn’t quite have enough for the whole picture, and as a result, he’s dead wrong.
Satou doesn’t take Shouko to the apartment where she lives with Shio; she takes her to her aunt’s apartment. Her aunt turns out to be very much alive, and the cops are at her door answering reports of a “suspicious smell” emanating from the apartment.
Satou’s aunt may be alive, but to the horror of both Shouko and the cops, she’s completely whacked out of her gourd. Seiyu Inoue Kikuko, a grizzled veteran of anime who’s played dozens of mothers, balances the sweet kindness of her voice with an underlying malaise.
Everyone who enters her apartment, and sees horrid room in which she sleeps, immediately wants to leave and take a shower. But before the cops can leave, having found nothing law-breaking, she literally jumps on the male cop, senses he’s lonely, and tells him he can do whatever he wants to her and she’ll accept it—sex, violence, violent sex…anything.
This, Satou later tells Shouko, is how her aunt considered “love”, being a receptacle for whatever other people wanted to give her, good, bad, and ugly…all of it. And she’s never changed, and likely never will, as the cops (and you could say society at large) are neither properly equipped or empowered to “do anything” about her.
The female cop manages to wrest her partner away (and turns down the aunt’s invitation to her), and then turns to Shouko, who she also senses is “lonely” and is looking for her “prince.” Satou comes between them and ushers Shouko out of the apartment. Halfway to walking her home, Shouko expresses herself honestly; that she thought Satou’s aunt was hella weird.
When Satou asks if, now knowing the woman who raised her and how she sees love, if Shouko will still be friends with her. When Shouko hesitates to answer, Satou tells her they can go back to being “just friends at work.” and leaves. Shouko wanted to know the truth, and she only got a small taste, and it was way too much, but she’s still ashamed.
After shedding her tail, Sensei, with some properly dominating language, Satou leaves her aunt’s apartment’s front door, marked 305, and walks up to her apartment with Shio, number 1208, where she continues her Happy Sugar Life, untroubled by what went down with Shouko.
But then we flash back to the rainy day she didn’t want to go home to her aunt anymore. Someone chatted her up, invited her to their apartment (1208), and asked her to model for them (they were apparently an artist). Now we know who she murdered: that artist and 1208’s previous occupant.