Densetsu Series

The story follows Kuu Sorakara, an unfeeling boy who always puts on a mask of emotion to hide the fact that he can never feel any emotion. One day, a scream was heard all around the world, killing a third of the human population. Six months later, Kuu confesses his unfeeling disposition to a doctor during a check-up, thinking nothing of it. The doctor, however, has ties to an organization known as the Earth Eradication Army, a force fighting to destroy the Earth itself, as it is trying to terminate the human race (the scream being caused by it).

The Army deems the boy to be a perfect candidate to become a hero, logically determining that since Kuu is unfeeling, he can never have any weaknesses born of human compassion, and therefore can become the best fighter against the Earth. And so, they send Kenka Kendou to kill Kuu's parents and siblings, and also dispose of every one of his acquaintances, no matter how insignificant their interaction may have been, in order to "sever all of his attachments to his old life." Thus, Kuu joins the Earth Eradication Army, having nothing better to do.

(Source: Nisioisin Wiki)