Kiba no Tabishounin: The Arms Peddler

The world has become a desolate, lawless land where bandits roam free, innocent people suffer and demons lurk around every corner. Sona Yuki is traveling with his family when they are attacked by bandits and his entire family is murdered and he is left with a snake mark on his hand. As he is about to die, a women comes up to him and gives him an option: continue to live in this harsh environment or die peacefully now. Yuki chooses life and Garami, the woman, gives him a second chance at life in exchange for his servitude. Garami is a weapons merchant and a gun for hire that travels from town to town for clients. Yuki sees how brutal and unforgiving this world can truly be.

(Source: MangaHelpers)