To unlock The Truth trophy in Assassin's Creed Liberation Remastered, you will need to kill all six Citizen Es in the game.
You will kill two of the Citizen Es as part of the main story. The first will be in Sequence 2 of Memory 6. The second will be at the very of the story after seeing the alternative ending.
The other four you can find in free roam. You can find their locations in the video below (credit to Powerpyx on YouTube):