In order to unlock this, you have to spare the life of Sean at the end of Mission 2 “The Club”.
To spare his life
DO NOT kill him in his second phase with a takedown, instead wait a few seconds until the prompt disappears and break his Structure again and you will see the option to spare his life with
. An alternative cutscene will play at the end and you will unlock the achievement.
Tip: Blocking and performing attack fill the bar to break your opponents Structure. After skipping the opportunity to perform a takedown during the second stage of the fight, it may be safer to block attacks to fill your opponents structure bar than attacking, however if you monitor their health you should be fine to throw a few attacks in.