Mercy ShotG 10

To unlock this achievement you simply need to kill one of your community member that has contracted the disease Black Fever. During the course of the game the church pastor William Mulroney will come down with the illness and while you are out exploring Alan Gunderson will kill him to prevent the pastor from turning. The disease is contagious and Alan will also contract Black Fever which will trigger a mission called " Mercy Shot ". Once you have the mission ask Alan to go out on a run with you, you will head towards a quite spot, and once there will have the option to shoot Alan. Put Alan out of his misery and the achievement will be yours.