Aku no Kyouten

A teacher named Seiji Hasumi is loved by his students and respected by his peers. However, his outward charm masks his true nature. In reality, Hasumi is a psychopath who is unable to feel empathy for other human beings. Specifically, he has a severe antisocial personality disorder. Because of his mental condition, Hasumi chooses to deal with problems like bullying and overbearing "monster parents" the easy way, by systematically murdering his students/parents.