Holy hell, that got dark for this manga. It does seem to be maturing with the characters and reader over time, though.
I first thought wicked Shiori was the manga jumping the shark, but I suppose it eventually became a little interesting.
I've been in a similarly dark place before. It stretched on for many years, too. When you lose your meaning (Shiori must have spent a lot of time planning, even living for her fiance), not just a lover, it can get really, really dark. Maybe it's foolish, as someone says. We're all different and respond to things differently. But, yes, in the great scheme of things, one is likely to have clarity and see it's foolish if they survive. No, it's not worth it. At all.
I wonder if it gets more pathetic here, though, as it sometimes does--perhaps Shiori wants to make a display and even cause guilt. That is one of the worst forms of manipulation. But maybe it won't go there.
I still hope Shiori changes somewhat for the better and has a happy future.