Just like Markiplier said.
Even if this is the last FNAF (its not, enough evidence of a sequel), the discussion and theories may go on forever.

If you Ask me, the Murderer was Neither Phone guy or one of the Playable guards.

phone falls out because he died in Fnaf1, while the killer was still alive at this time, oh sure, we could say he faked his death, but it's mentioned nowhere and thus makes this just straw grasping. also, he knew the suits are waaay to dangerous to use. something the killer didn't know.

mike falls out for me because he is, chronologically, the newest guard, and i don't think he even knew that there was a yellow spare in the back, and even if, chances are he has no idea how to operate the Hybrid-suit.

same for the fnaf2 guards, Fritz was fired on day one, don't think he had anytime for finding out about any extra suits. and jeremy was either the Bite victim or wouldn't have any idea how to use the suit like mike, all he knew was that there was "A spare suit in the back, a yellow one"

What i DO know is that the Killer was a guard, he must've been working at Freddys for a damn long time, and probably wasn't fired in all that time, he knew where the spare was, and he knew how to use the hybrids, what he didn't know was the suits gonna kill ya if you tried to wear them thanks to the shoddy springs. and Phone guy explicitly said NOT TO touch the hybrid suits because of an…Accident in a sister location.