Ubisoft has added an alternative ending to Far Cry 5 and it makes the game incredibly short. During the introduction sequence when you move out with the local sheriff and federal marshal to arrest cult leader Joseph Seed at the Eden’s Gate compound.
When confronting Steed you are asked to cuff him and a button prompt appears to carry out the action. To get the secret ending do not comply and do not press the button. If you ignore the prompt, then it’s effectively game over.
The video below demonstrates the full procedure to make it happen.