There are many theories like that no other than Jeremy himself was the killer or that it is the purple guy. Some people also say that night 7 might come before Night 1. Now this theory is based off a real life murder from a Chuck E Cheese account ( The Nathan Dunlap Murders) that is supposedly what the FNAF games are based off where a guy worked there, got fired, then came back and killed 5 people who were working late in the building (though one apparently survived). This might mean that if Jeremy is the killer, you did night 7, got fired, then came back and decided to get some sort of revenge and killed the five kids. Or, in regarding the purple guy who is also supposedly the one who kills the kids, did it in that way. "Lost to this monster, the man behind the slaughter" (from the song; It's been so long by the Living Tombstone again) some people think this links to the Purple guy being confirmed as the murder and in being the phone guy in FNAF 1.
But now there are also thoughts on which suit the killer used, a lot points to Golden Freddy, but some people also think it might have been the original Freddy (or Freddy 2.0) for some particular reasons even though the FNAF 2 game states that someone had stolen a ;yellow' suit.
