darkSpyro

Here's my viewpoint on it.

During the events of FNAF2, a murder of 5 children takes place, and the killer uses the Golden Freddy suit. Phone Guy mentions on Night 6 that "Someone used one of the suits. We had a spare in the back, a yellow one, someone used it...now none of them are acting right." More than likely meaning that the old suits were where the children went, and also explains Golden Freddy's appearance on Night 6, as well as the Toy Versions being more active as they supposedly have facial recognition and are tied to a criminal database. They know that it was an adult, and a security guard, however they see all adults as purple, much like they see angry kids as red and happy kids as green. And when Phone Guy says the day shift became available on Night 5, he is most likely referring to the fact that one of the workers at Fazbear's had committed the murder. They had most likely been arrested, or perhaps they had framed someone else, though this is unlikely. And Phone Guy also says "Uh, we're going to try to contact the original restaurant owner. Uh, I think the name of the place was..."Fredbear's Family Diner" or something like that. It's been closed for years though, I doubt we'll be able to track anybody down." possibly trying to get all of the legal stuff done, or just seeing if the murderer has worked there, though I know little to nothing about that line.
Now, we very well know that the murders are Purple Guy's doing. It is evident in the Death Minigames, which also show that he murdered another child at FredBear's, who presumably possesses The Puppet (or as I like to call him, Howard). This is at the time of FredBear's and after the Golden Freddy and Springtrap suits are deemed deadly and hazardous, which leads to the actual animatronics known as Freddy, Bonnie, Chica, and Foxy. This then leads to FNAF2, where the day after Night 6, Jeremy is told to guard the people from the animatronics, and it is presumably Mangle that attacks him and damages his frontal lobe or removes it completely. Jeremy's paycheck is sent to him after all of the medical work and lawsuits are over, which explains why he is given it in November when he was working a summer job. Fritz Smith takes over on Night 7 before all of the Toy animatronics are disposed of and the old ones are moved to a new location, being the place in FNAF1.
Now, we're in FNAF1 time. The security guard is now Mike Schmidt, and before I get into my theory about him, I'll go over the timeline. Phone Guy either worked the previous week of the 12-6 shift or the 6-12 shift of the current week, though I think the former is more likely. He, unfortunately, dies his fourth night regardless of week. It is unknown what suit he is stuffed into, if at all, though we can infer he was stuffed into SOME suit based on what he says on Night 1, about people looking like endoskeletons. However, he could be wrong. He has no way of knowing what the animatronics, or rather, the children's souls are thinking. Perhaps they, like I had said earlier, see all adults as purple and they know that their murderer was purple, therefore they want to kill whoever did it, but they have no way of knowing the difference, which is why they continue to hunt you after Phone Guy dies. You finish your week, stay an extra few days, get fired, leave. 30 years later, Fazbear's Fright opens, however, a few years after the FNAF1 pizzeria closes is when Purple Guy returns, dismantles the animatronics, flees to the Springtrap suit in the safe room, and bleeds out, resulting in death. Night 1 of FNAF3 is when the Fazbear's Fright team goes to search the place for any "relics." On Night 2, Phone Dude (lol phone dude) says that they did in fact find an animatronic, that being Springtrap possessed by Purple Guy. Events of the Third Game happen and the timeline ends there. So far.
Now, for that Mike Schmidt Theory. Mike was around during the events of FredBear's, and killed a kid. He most likely did not work at the place during that time. The staff likely did not know who he was. Fast Forward to FNAF2, where Mike works the day shift. He commits the murder of the 5 children at an unknown time, most likely before your shift on either Night 4 or 5. For this, he either gets arrested, or frames someone. Now, since murder causes life in jail, (I believe, could be wrong there) Mike had probably framed another employee. 6 years is too short of a time to be in jail for murder. So, he still works at the establishment during their move to FNAF1. He then works the Night Shift after Phone Guy. There is a possibility that the animatronics want to kill Mike, and this is why they continue to attack after Phone Guy's death, but this could go back up and collide with the fact that the animatronics see all adults as Purple. Mike gets fired after a week, ending on Night 7. A few years pass, and now the Pizzeria is full of rats and leaks. Mike comes back to dismantle the animatronics in fear of them haunting him because of his murders. However, he gets cornered, and dies in the Springtrap suit. FF to FNAF3 and his body is 100% decayed, with his skull clearly still inside the suit head, impaled by the metal parts. He follows BB's voice because it sounds like a child, and he still has that murderer instinct in him. This is just a theory, (a GAME TH*stabbed repeatedly*) however, I believe it is a pretty accurate one.