When you gain control of Shel, go theough the door to the kitchen. Then take the door into the pantry and the next door outside. When propmpted, go and check out the walker in the far corner. Then come back inside.
You need to make a decision about whether to kill someone or let them go after they are caught stealing from the group.
After that you will be presented with another decision. Either kill Stephanie or Drive away. You will get the achievement when you make the decision.