Gareki no Rakuen

From Boys On Boys Love:

A skilled investigator operating outside the law infiltrates a prison island where he meets a smart and cheeky boy who seems to have an air of danger about him. Ten years have passed since that day. Now the boy who was given the name Yuuto works along Shigure, who rescued and raised him. All along, Yuuto harbored secret feelings for Shigure, and the deep love that he couldn’t express only made him more dangerous. Soon he started to think that he wanted to die for Shigure. Then he seemed to get the chance when a wild shoot-out broke out on the streets...