The One You Should Not Have Killed

The One You Shouldn't Have Killed is the mysterious, mostly unseen, character of the non-canon Five Nights At Freddy's game, Ultimate Custom Night. He is responsible for the players seeming inability to die and the fact that the nightmare animatronics had physical forms. It is also possible that he is the cause/creator of the purgatory-like dimension the player is trapped in.

Ultimate Custom Night
It is most likely that the one you should not have killed is the likely leader who is commanding the anmatronics in the dimension that the player is trappded in and has the inability to die.

Triva

 * It is most likely that the one you should not have killed is some sort of leader.