Hello what i remember is. The settings of the manga was post apocalyptic where humanity moved beneath the ground to survive, the monsters of the manga are infected humans of some kind who can only be killed by fire i believe. the mc has an arm blade which he sets on fire so he can kill the monsters (he stil has both arms). he's part of some organization that protects the villages from these monsters. he is a cold mc who doesnt show much emotion and he used to have a brother who was killed, and he is also trying to hunt down the killer.

Hope someone can think of something, i will continue my own search in the meantime.