No redeeming qualities in this one. It's pointlessly edgy, the premise of "killer that only kills other killers" is a desperate bid to portray the murderer protagonist as at worst morally dubious. None of characters feel human, and it's not because they're sociopaths/psychopaths, but because they have no actual character development or any depth - what you see is what you get, with no nuance whatsoever.
Female protagonist has no survival instincts, lets herself be led by the nose by (at least) 2 serial murderers. Male lead is very touchy with her in public and even worse in private, which none of feMC find creepy despite her being a schoolgirl and him being her teacher.
The gory bits are short, unfulfilling and pointlessly cruel. There's a certain sublime art in properly drawing gore, it has to be sticky and visceral, but here it's just... flat for lack of better words. It's like you'd give a prompt to a language model. There's just a lot of blood and skewered bodies and it's obvious that mangaka has no sense of scale when it comes to body counts in murder mysteries. If there's a tag for unrealistic fighting, then there should be one for unrealistic violence.
I claim it's an edgy shoujo, because by the time I dropped it, it had two creepy serial killers with almost the same personalities, competing for feMC's attention and being very touchy with her. It's obvious that the concept of "first murder" that is thrown around feMC is an allegory to being deflowered, but it's as subtle as a brick through a window.
All in all, it reads more like a power fantasy wish fulfillment than an actual murder mystery/drama. A low IQ slasher for a low IQ reader.