I have a theory... I think Purple Guy is Springtrap. Here's why:
After you die, there is a minigame. In said minigame, you play first as Freddy, then Bonnie, then Chica. See, the Purple Guy uses the safe room mentioned in the phone calls you receive that animatronics are oblivious to, which Purple Guy uses as a way to safely destroy each animatronic. But the Purple Guy hadn't planned on the Marionette's spirit being able to enter the affermentioned safe room, which means he is surprised when he sees the spirit. The Springtrap suit he uses was not supposed to be worn at all, because if you even breathed hard (which apparently Purple Guy did), then the springlocks would release, causing the person inside to bleed out and die, which Phone Guy tells the player in one of the night's phone calls. Now, I think this happens before the events of FNAF 3. Why?
Simply put, in the game, the animatronics are nothing but phantoms without bodies to possess. This would explain why Springtrap's movement are more human-like.
But then I know you're wondering: If the murderer died, then why are the spirits of the children still caught in purgatory?
Because Purple Guy is not dead yet.
They apparently hadn't planned on him being able to possess the suit that he died in, or the Marionette wouldn't have lured him into the suit in the first place.
But then another question: Why are the spirits helping Purple Guy murder you? Why would they help their killer? The one who got them caught in this hellish realm, unable to move to the afterlife?
Well, I have a possible answer to THIS, as well. Maybe the spirits don't even know Springtrap is the Purple Guy (if he even is), and just another one of the Purple Guy's victims. And since they don't possess that little facial recognition software they had when they were in their previous bodies, they can't connect to the database they need to identify a person. This provides Purple Guy with a way to continue his job of murder. And the bad part about it is: The animatronics who so desperately lust for revenge. It's kinda sad. Just think: The person who killed your brother is now using you to continue his duties. The thought is quite... disturbing.
This story can also be proved because when you brighten the image on the End screen, there are words that say:
"THEY WILL KNOW THE JOY OF CREATION"
Is there another game on the way? I dunno, but Scott is one sneaky mofo!
Please, tell me what you think of my theory, and any holes you may have seen. I need all the help I can get.
(Oh, wait! Forgot to do my Game theory outro!)
But hey, that's just a theory! A gaaame theory! Thanks for reading!
*Post cheesy fnaf meme, followed by Foxy booty joke here*