Help with finding this manga about a family of killers who wield scissors?

Question

I can't completely remember the story, but this is the main protagonist:

She is a high school (?) student whose best friend tries to kill her after she's drugged/hypnotized into killing anyone who is a murderer, but — surprise! — she's in fact a murderer who hasn't yet "awakened" to her murderous tendencies.

Another serial killer that wields giant scissors that come apart (it's one scissor by the way) comes along and finds her and tells her that she's now his "little sister" and she's "part of the family", to which she accepts. Apparently, he got lonely being a ruthless serial killer and starts gathering serial killers to form a family. He defends her against people who are hunting serial killers down, and kills the girl's family and drugs the city and that's as far as I've got.

Answer 1

You're probably looking for Zerozaki Soushiki no Ningen Shiken. This is a manga adaptation to NisiOisin's Ningen series, which is a spin-off from the Zaregoto series

Iori is a high school girl who doesn't know what she wants to do with her future, and tends to run away from things. One day, she is attacked by strange dead-eyed people on her way to school, only to be saved by an equally mysterious long-haired man. Later that day, her friend at school, Jion, seems to fall under the same murderous influence. To her surprise, she finds herself capable of defending herself from his violent attack. Afterward, the long-haired man she met in the morning, Zerozaki Soushiki, arrives once more, this time to invite her to become his little sister. What has Iori become involved with, and will she be able to survive this change to her life?