Murderous secrets are uncovered in the final episode of the "Watadamashi" arc. I've got something like two pages of notes for this angle of the mystery so let's dive in.
It's important to note that the last episode and this episode take place mostly in the same 24 hour period. Keiichi has arrived in class after walking with Mion and Rena and having his strange encounter Rika. However, even though they just spoke moments ago, Rika isn't in class. Satoko remarks suspiciously that she saw Keiichi talking with Rika while Mion mentions seeing speaking to someone that looked like a construction worker. The entire class heads out to look for her and two central areas are framed to grab our attention: an outhouse and an incinerator.
Keiichi begins to open a door to the outhouse but is interrupted by Mion who says she saw something on the roof. Keiichi goes up on a ladder to look only to have Mion have a breakdown where she swears that Rika and the Three Families are responsible for Oyashiro-sama's curse and she's going to put an end to it. She shakes the ladder incessantly but before Keiichi can became a famous Angel Morte pancake, Chie-sensei brings them back to reality.
At this point there are five people missing within Hinamizawa since the festival, Miyo Takano, Rika Furude, Kiichiro Kimiyoshi (the mayor), and Tomitake Jirō. Keiichi, Rena, and Mion walk home after school and Keiichi intends to confess to going into the storehouse until he notices they are being watched by men in a van. This is the same group we saw in previous arc unloaded boxes into the Irie clinic. Once Keiichi gets home he receives a call from who he believes is Mion but I think it's suspicious that the caller never really confirms her identity. Keiichi agrees to meet her at the waterwheel and the two head to the Sonozaki mansion.
This is the first time we've seen Mion's home and it adds a whole new layer to the character. Up until this point we knew her as a member of one of Hinamizawa's three families (Sonozaki, Furude, Kimiyoshi) but her personality doesn't really match the typical "elite family heir". Seeing the estate hammers home that despite her joshing and free-wheeling, Mion has a legacy to take on. Once inside, Mion leaves to get tea and returns in a white kimono. This is a detail that's going to be important going into the climax. Traditionally, we only see white kimonos in pop culture for a few reasons:Marriage (multi-layered, red accented) Part of purification ritual, usually worn before/after bathing Burial
The Mion that Keiichi encounters here is nothing like the girl who nearly killed him earlier in the day. Keiichi makes his confession about what did during the festival and Mion forgives him, but also tells him that a lot of people know what he did and want to hurt him. She posits that the once coincidental deaths that took place after the festival have been co opted by the Three Families of Hinamizawa to carry out murders under the guise of obeying Oyashiro-sama and as the heir to the Sonozaki family, the only family that still maintains power in the village, she plans to put a stop to it. While not elaborating, she cites Satoko's family as victims.
Keiichi follows Mion through a series of darkening underground rooms that reveal a modern torture chamber and jail that has never been used but is nevertheless available for the right occasion. Further in is "her uncle's" safe room that has a one month stockpile of food and monitors to observe the security footage throughout the mansion. She locks Keiichi in a cell to protect him from what she believes are murderers sent by her family and takes a gun to confront said intruders. Mion breathlessly confesses her love to Keiichi and thanks him for treating her like a girl by giving her the doll. With that, she takes a revolver and leaves.
Keiichi is only jailed briefly after throwing himself at the barred door a few times but he doesn't actually make it out of the larger room. After passing out from probably giving himself a concussion, he's rescued by Ooishi and taken to the hospital. When he's released, the detective is all but elated to tell him that everyone is dead.
A well in the underground area contained the bodies of Oryo Sonozaki (Mion's grandmother), the mayor, and Shion. Rika Furude was found in the outhouse septic tank on the school grounds. Satoko and Mion's bodies were found dead, presumably due to gunshot wounds, in the hallway of the mansion. Another quick note is that when Keiichi escaped the cell and looked at the security cameras he saw the men from the van.
With the exception of Satoko, Miyo, and Tomitake, the dead comprise every known member of the Three Families. A possible conclusion is Mion achieved her goals; she ended the curse by taking out everyone she thought was responsible for its existence. Of course, it's impossible to ignore Mion's own mental state at the time as well. We know that Rika is actively trying to thwart the curse throughout the timelines so her conclusions can't entirely be trusted. Neither can her identity, either. Keiichi never physically sees the well, but Ooishi's admission about the gunshot prompting the detectives to find Keiichi implies that he was found not long after Satoko and Mion's confrontation. The bodies in the well were likely there before Keiichi was locked up but we also haven't physically seen Shion and Mion in the same place since the festival. There's a whole lot of questions about what was really happening after that.
This arc's conclusion is significantly different than the original, whereas the last arc's changes seemed less so. Even with my knowledge of what truly went down in the previous series, I'm skeptical to assume the same circumstances were repeated here because of the doll and Mion's composure. Does GOU plan to use the same Question Arc/Answer Arc format? I can feel myself actually getting antsy to find out.