• To get this achievement/ending Rayne should be the killer and he has to kill you in the end.
  
• The killer is chosen randomly every time you start a new game.
  
• After the first playthrough you can skip all scenes.

1. Start a new game;
   
2. After talking to Violet, Hail a Cab and Visit the Chief, ask all questions;
   
3. Visit Rayne and ask all questions;
   
4. Visit Lexie and ask all questions;
   
5. Go to your room, shapeshift into Lexie;
   
6. Visit Rayne. If the first question is "HOW ARE YOU?", RAYNE IS NOT THE KILLER.
   
7. Go to main menu, start over.

When you don't have the "HOW ARE YOU?" question, Rayne is the killer, so continue the game.
Towards the end, when the Chief asks you who should be arrested, chose Violet or Zak, BUT NOT RAYNE.

In the scene when Rayne talks to you, holding a knife, chose SHAPESHIFT INTO RAYNE.
Continue the game until the achievement pops up.

CREDITS: Dresden N7 & QuanticJustice @ TrueAchievements.com

1. Exit the game completely;
2: Extract the ZIP archive in C:\Program Files (x86)\Steam\userdata\YOUR USER ID\898650
3. Start the game, select RESUME;

There are two important choices until the end:
4. Tell the Chief to arrest Violet or Zak, NOT RAYNE;
5. In the scene with Rayne holding the knife chose Shapeshift into Rayne.
6. Continue the game until the achievement pops up.

You can also get the Look Mum I'm A Detective achievement, by repeating steps 1-6, if in step 4 tell the Chief to arrest Rayne.

CREDITS: IBadDriverI @ YouTube