This is one of the endings of the base game.
*** Spoiler - click to reveal *** In the final room, you need to put the dead janitor on to the pedastal at the top, near the exit. Hit the switch and an arm will scan the janitor, opening the exit. TO FREEDOM!
To get the other ending (and the other trophy), just hit Continue from the Main Menu. This will take you back to the last checkpoint. Simply follow the instructions in the other trophy guide: