To unlock this achievement you need to:
- Play through making all negative morality decisions
- Play through making all positive morality decisions
- After completing your positive morality playthrough, you can reload the final chapter from the main menu and pick the oppisite character. You can check the status of this in the main menu under "Extras" and then "Co-op Choices". The spoiler tag has details about the final decision.
You will need to make the decision to kill the bad guy when you playthrough your negative morality playthrough. Then on your positive playthrough have Rios kill Salem, and have Salem kill Rios.