Kiba no tabishounin - the arms peddler manga

Kiba no Tabishounin - The Arms Peddler summary:

The world has become a desolate, lawless land where bandits roam free, innocent people suffer, and demons lurk around every corner. Souna Yuuki is traveling with his family when they are attacked by bandits. 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. Yuuki 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. Yuuki sees how brutal and unforgiving this world can truly be.