Shockingly good story, I'll just briefly mention the art style and character design (both in writing and appearance) because both of those are also really good, but not my main take away. This feels like a "Joker" story done right, it doesn't really come off as cringe or haha society lol, but it feels like it has a lot of depth in the actual descent into a form of madness (spoilers from here on out) I loved the detail of the acceleration of Mercredi's violent tendencies as soon as she hears Mardi's story. The character you've grown to love just disappears in an instant and almost just as quickly everyone feels against her. And at the end when there's maybe a shrivel of hope for her redemption it's to late. It might be harsh but now I can't imagine the story without it.
There's so many other themes I can see and feel from this to, and I have to congratulate you for making such a good novel while juggling all those different elements that make Mercredi more likable to more detestable in a viable manner. 10.1/10!