Satsui no Senki

Mikoto Rikudou, your average secondary school kid, lives an ordinary life and doesn't think much about the unexplained appearances of figures called "demons" in the city. This is all turned upside-down when he finds the most timid girl in class is one of them. He is caught in the middle of a battle between these creatures and ends up becoming one himself… only to be told he will have to kill other people using his new powers or lose his own life instead.

(Source: MU)