Misery porn that insists upon itself. I think that was the point. When I was trying to process my thoughts on this I tried to estimate a percentage of how many scenes in this game were misery focused and I'd say 85% of them, such as her torturing herself, her uncle, her grandmother's corpse, Satoko, Keiichi, and arguably her own sister. That's not even getting into watching her fall in love with what you know is a man who's going to die/disappear soon, so that's not a particularly compelling plotline to begin with either even though it's supposed to be the closest thing she has to a sympathetic motivation. I don't think it's the first Higu ep to have a downer ending, in fact I'd say all of them before this do, but the thing about those was that the protagonists were compelling people I could sympathize with. I cannot do that with Shion. She is a psychopath that I struggle to think of any redeeming qualities beyond surface level ones. WhenI watched Keiichi commit murder in Tatari I felt like I was right there with him, mostly sympathizing with his goal. For the entire second half of this, I felt like I was being forcefully dragged along by Ryukishi until I could hurry up and finish it so I could read future, hopefully better episodes.
The bittersweet parts were probably my favorite thing in this arc. Seeing ways that things could've gone better and the moments where Shion truly regretted what she'd done were pretty impactful, but I don't think they were worth sitting through the misery of everything else.
tl;dr, Ryukishi made something that was made to make me feel like shit and yeah, it did that.