This is a decision based trophy unlocked early on in chapter 11
You will move forward and find a character injured on the ground. Your choice will be to either end his life by shooting him, or to walk past and let him die a more painful death.
For this trophy, walk past and keep moving on
This trophy will not unlock until the next checkpoint so you'll have to make the opposite choice on a second playthrough.
Note: if you missed
this can be done shortly after if you make this decision.
Walk forward and you'll find an oryx eating a dead person. Shoot it with your single bullet.