Read manga: Dead Soul Revolver

manga
Summary:

Kagami Kenichi meets a demon murdering a woman while taking a shortcut home from school. He is rescued by Tsukishima Miyu, a gun-wielding girl. She explains to him that the demons were once humans and the only weapon against them is guns. He soon discovers his classmate, Hirai, is a demonized human when he is attacked by him. Later on, Miyu makes a contract with Kenichi and uses his soul energy to summon a "Demon Exorcising Weapon" and kill the demon...From Baka-Updates:When a person who under goes extreme depression/anxiety, loses control, a person will become a demon consumed by darkness. Luckily there are those devoted to hunting those demons.Boy meets monster while taking shortcut home from school, is rescued by a girl with a gun. Monster gets away. Later on, girl makes contract with boy and uses his soul energy to make a big gun and kill the monster...