Waltz

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)