This trophy will unlock if you choose to kill Lord Regent during mission six. This trophy CAN be missed should you decide to broadcast the Lord Regent's crimes rather than killing him.
This mission sees you return to the place where the Empress was assassinated. You must infiltrate the heavily guarded tower and eliminate Lord Regent. You will begin swimming in water and must make your way upwards to the main courtyard, which is very heavily guarded. If you stick to the left hand side you can traverse it without being seen, which will benefit you because Lord Regent will retreat should you be spotted in the courtyard.
Follow your waypoint into the large main chamber, and then Blink the chandeliers up and to the left to make it out onto the roof. Kill/avoid the guards up here, and then Blink into the window on the right side of the building ahead, which will lead you to Lord Regent talking to somebody. Deactivate the wall of light, enter the room and kill Lord Regent to unlock the trophy.
Please Note that this trophy and Political Suicide can not be earned at the same time. If you wish to earn both trophies in one playthrough, make a manual save once you enter the large main chamber with the chandeliers. Choosing to assassinate the Lord Regent will also negate the Poetic Justice trophy should you be attempting that too.