Psycho no Sekai

A girl was killed. Her body was cut in half. The culprit was not found, and her death was soon forgotten. After all, in this world, the weak were trampled by the strong. But 12 years later, the girl's childhood friend, Mamoru Inubo, begins to take revenge. He wants to destroy the murderer that the world has overlooked with his own hands. Inubo begins his tough journey by finding a "partner" that he has been lacking in many years of planning. Revenge suspense of rebellion!

(Source K-Manga)