This achievement is story based so it is unmissable however I will post the quickest way to get through the chapter if you don't want to spend time exploring for back story.
This chapter is super short. Once you gain control speak to Doug to go outside and find the walker trapped under a pole (Lee's brother who has the keys to the pharmacy.) Try to use the remote on the TV store across the street which triggers a cutscene of Doug programming the remote and turning on the TVs. Now hit the lock on the gate with your axe, open the gate and pick up the brick directly infront of you on the ground. Throw the brick through the glass window at the TV store and then run to Lee's brother. Follow the onscreen buttons to kill his brother, grab the keys and go back to the employee room and open the pharmacy door.
The achievement will pop after you kill your brother.