From Girls' Generation Scanlations:

There's been a string of mysterious deaths, where the victims are torn up for no discernible reason. One day, the situation escalates and Homura Kei's computer class of 36 people is slaughtered. Due to Kei's delinquent behavior in skipping class, she ends up as the sole survivor of the massacre. When she goes home that night, a hacker called Yuon comes to her door. He claims to have also lost someone to the mysterious murderer, and he has information that Kei does not. Apparently, someone has created a program that can affect humans through the image of a blue butterfly on a computer screen, and that someone is using the program to murder people. Kei and Yuon will have to track down the murderer, before whoever it is decides to finish them off!