"The Perfect Crime" is a Most Wanted Side Mission in Batman Arkham Knight. You must find and scan 6 mutilated bodies. Completing this mission unlocks the "The Monster Machine" trophy or achievement.
Play through the main story to get access to all of these dead bodies. Some of them aren't available until much later in the game. After finding all of them you must track down the serial killer Professor Pyg, defeat him and lock him up in GDCP.