Right in the beginning of Chapter 9, rappel down and continue on. After the cutscene, will be surrounded by snipers. You can either follow their orders and shoot the civilian or you can disobey and shoot the soldier.
For this achievement, you'll have to follow their orders. Do not open fire on them, and instead shoot the civilian (hanging on the right). The game automatically saves after your decision, so you will have to get the achievement for the other decision using chapter select, or another playthrough.