you know, by the synopsis you'd expect this manga to be about a protagonist confronting the duality of trying to save an innocent girl from being bullied despite he himself being bullied (though for some reason it's always something akin to hardcore torture in manga) by the girl's father, right? You'd be wrong, the MC is the biggest and most disgusting villain, even more than the girl's father. And, you know why this plot device falls flat unlike with the Joker movie? The people target to the Joker's atrocities always deserved it someway or another. So, when you've established that your main character is someone who's suffered and who deserves our pity and who we root for (just like with Arthur) but then you change him to be a complete villain with no redeeming qualities, a psychopath no different from the rest of the villains, you have yourself a bad story. That's simply because the main victim of all of this doesn't deserve it. All the goodwill we've had for him gets washed away as if it never was there to begin with. The MC transforms into a bad guy that does bad things because he's bad. As someone else here put it, this is the equivalent of NTR hentai.
In the end this is just an edgefest. Sure, the “endgame” is probably going to be satisfying in a very morbid and twisted way, kinda like the Joker. But, the author could've told a story just as impactful and harsh without being so over-the-top edgy. Had it been more realistic (the bullying and the personal relationships between the characters, in general), this would've convinced a lot more readers and had a bigger effect.