I normally don't get involved in these discussions, but I do love a good murder mystery. Consider this, homies:
In game #1, it's established that the old animatronics attack you because they are possessed. This is later supported by the minigames in game #2. It is also revealed that the possession occurred even before game #2, which explains why they attack you even back then.
Now, the people saying it's phone guy are basing it off the fact that the murderer was pretty much definitely a security guard. Their conclusion is wrong, but the premise is sound. A security guard would have ample access to the behind-the-scenes areas that the murderer was shown to be using.
With this in mind, I have one more bit of evidence I'd like to share. One that I'm surprised I haven't seen anyone else bring up. The '87 animatronics were never possessed, and yet they attacked the player anyway. Remember, these things were brand new at the time the game started, and were said to perform their pizzaria jobs extremely well. They had absolutely no reason to go on the offensive.
Or, it would be more accurate to say that they had exactly one reason. This is from the Night 1 phone call in game #2:
Criminal database. Designed specifically to detect child predators. This isn't just a random bit of dialogue, either, it was featured heavily in the game's advertising before release. Scotty really wanted people to know that these 1987 models were top notch. Yes, it's mentioned by phone guy that same night that there was another guard that quit due to "conditions," but he also only says that the robots would "walk around" at night. He makes no reference to the new animatronics being aggressive, besides the puppet (who IS possessed). He doesn't even use the "stuff you in a suit" excuse from game #1, he just says that they walk around.
To me, it seems pretty likely that the game #2 guard is the killer. Being a current employee at the time somebody "used one of the suits," he would have definitely had the opportunity. Hell, the death minigames in game #2 even tend to favor a map layout identical to that of the game #2 restaurant. Whoever the murderer is, he was definitely on the staff during the events of game #2.
There's also one more piece of evidence. This one is unmistakable. From the night 6 phone call, game #2:
Not only did the guard return on Night 6 despite explicit orders not to, but he was heavily implied to be at the scene during "The Bite." I really don't see how everyone missed all this. It seems to be pretty obvious to me.
Jeremy Fitzgerald: J'accuse.