Maoh: Juvenile Remix

Suzuki, who was once a law-abiding teacher, joins a criminal organization called Reijou in order to avenge his murdered wife. But his attempt is thwarted by mysterious man who comes to be known as the "Pusher" because he shoves the criminal into the path of an oncoming car. Suzuki decides to pursue the Pusher. This mystery man is also being pursued by two other hit men; the knife-wielder Cicada and the assassin Whale, whose family is in the business of making people commit suicide. The story is written from the points of view from these three people and ultimately relates how their fates are intertwined.

A nameless high school dropout teenager turns into an assassin for hire. He is a complete loner who uses his knife to kill his targets without any emotion or hesitation. He kills anyone who gets in his way whether they be a complete stranger or even the client that hired him in the first place. He meets a man named Iwanishi after one of his jobs goes bad. Iwanishi nicknames him Semi (Cicada) because he apparently talks too much and offers to become his manager, handling the business side of assassination. But it seems like Iwanishi has some secret agenda Semi isn’t aware of.

(Source: MangaHelpers)