Looking for a manga

It is a Yaoi manga
it is about a guy (named kai if I remember correctly) who's girlfriend (named Maria if I remember correctly) was killed, he goes out looking for the killer and tracks the killer to a city. This city is ruled by two gangs and the police has no power here. He ends up with one of the gangs and finds out that the boss of the gang is the killer. He gets close to the boss in the hope to find the right time and kill him for revenge.