Tsumibito

As his life partner Airi is ruthlessly murdered by his once best friend Hisui, Izumi simply snaps. His life is no longer worth a thing. There's only one thing to do for the man he loved - avenge his awful death and follow him suit.
Using his connection to the Police, he quickly finds the culprit, Hisui and takes to pressing a motive from him. What he gets to hear though is utterly unbelievable to him...