Mercy ShotG 20

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.

If you happen to miss the opportunity to unlock the Mercy Shot achievement with Alan in the main campaign (like I did) or you don't want to do it twice, I would recommend saving it for your Breakdown playthrough, as it is a requirement to unlock one of the hero characters (Alan) and since there is an achievement for unlocking all the heroes, you'll have to do it anyway. While I was playing the main story, I did get the calls that the pastor was sick, that Alan killed him, and that Alan was sick. I messed up though by bringing Alan on a couple missions afterwards and leaving him out there and I didn't stay close to home base. I ended up getting the story line where you meet Alan in a secluded spot and he shoots himself.
If you decide to unlock it during your Breakdown playthrough it might be a little more difficult to get the mission to trigger but shouldn't be a problem if you do it right. First of all, you'll want to make sure that you do not have any type of medical facility built at your home base and you will not want to bring back any medical supplies back. Bringing back individual medications and snacks is okay just don't bring back any crates. You will not be able to unlock this achievement by trying to do it with any of the heroes, playable characters, or any member of your community that has full trust in you and is shown as being your friend. It's best to do it with new survivors that you find. If yiu don't see any on your map that need help, you can ask Lily to radio to any survivors and offer shelter to them. Now you'll want to try any get one of these members sick. The best way is to find a Zed Huht mission where a Bloater needs to be located and killed. If one of those missions aren't available, drive around until you find one. Once you do, run it over and make sure to stay in the car afterwards until all the Bloater has goes away. Your health will drain very low but don't worry, you won't die. Just make sure to have some meds with you. You can also run over survivors repeatedly or let them get attacked by zombies to help get them sick, just make sure you don't kill them. It shouldn't be long until they start to get sick. It might start as a cough or fever but should progress to them being gravely ill. After that happens, it still might take a little while for the Mercy Shot mission to appear. For me it took a couple days (real time) until it finally triggered. Since it can take a little time and since you won't have any medical facilities built, just know that it probably will spred throughout your community and many of your survivors might get sick. I would recommend to try avoid unlocking heroes if you can during this process since they are the best characters in the game and you'll want to keep them alive if you can. By the time Mercy Shot popped up for me, I had 16 community members, 13 of which were sick and probably half of which were gravely ill. I had 1 playable character and 1 unplayable that were okay. Then of course the last being Lily. You can still use playable characters that are sick however, their health and stamina will drain a lot faster, and it won't take long until their stamina is pretty much completely drained and they'll need to rest. After Mercy Shot was available for the first time, 2 more popped up right after. If more pop up afterwards and you want to try and keep that character alive, make sure to choose that they either have to leave the community (when I did they refused) or to hold out hope on recovery. would definitely recommend immediately building a medical facility, fully upgrading it is possible, and bringing back medical supplies right away after unlocking the achievement. If you are able to upgrade to medical lab, you'll notice that members of your community will start to get better. You still could lose some of your community, but even some that became gravely ill fully recovered. Even people who trigger the Mercy Shot mission can recover, just make sure to have those facilities built and supplies brought in.