I imagine there are
many other endings I haven't seen yet
But as for the ending that I believe is the true ending...
Move - WASD
Interact - SPACE
Sin info - E
Go to sleep - Q
Mute music - M
Interact with the people and buildings in the game and max out all the sins.
Use your tablet (Press Q) to read about the sins.
When you're ready, go to sleep (you'll get your last sin here) and see what awaits you.