Aku no Kyouten

Hasumi Seiji, who teaches English at a high school, is affectionately known as "Hasumin" by his students. He's popular, good-looking, kind to everyone, and efficient at teaching and solving problems. But no one knows that he's actually a manipulative serial murderer. He treats the school as a kingdom he rules. He manipulates others into trusting him and obeying him. But when there are people (students, teachers and parents alike) who cannot be reformed, he kills them off without mercy. Eventually, his targets include all the students in his class...