I have to gather my thoughts for a thorough analysis but since his introduction I've always felt like perhaps it's the puppet. Maybe he was the original victim but I also feel like maybe he's responsible for the other murders and not Purple Guy. He just seems too sinister to be more involved in the events.
-The crying children posters in FNAF 1 resemble him.
-The cutscenes in FNAF 2 seem to take place in FNAF 1's location which we know was also open prior to the events of FNAF 2. In these cutscenes the other animatronics become increasingly aggressive towards Freddy, it's me flashes as the cutscene ends. Then the animatronics are docile again but Golden Freddy is now in front of you, again, it's me. Finally the puppet appears, incredibly menacing, won't let you look away. It's me.
-In FNAF 1 Freddy is the most cunning of the animatronics, adapting to your play style. But when the lights go out it's him who attacks. And without the buzzing of the electronics in the office we can hear his jingle clearly, a tune almost like a music box. It isn't until the music stops that he attacks, sound familiar? (Not entirely sure the relevance here, I feel like maybe the puppet is good at hijacking the machines, perhaps making them more aggressive than usual. Or at the very least he's hiding in Freddy.)
-We have reason to suspect that Golden Freddy was a functioning suit, perhaps a spring lock variant. Either way we know he's hollow and can be used.
-In some of the hallucinations the puppet appears to be quite large, nearly endoskeleton sized. Perhaps he can stretch.
Also a bit of a separate thing but I feel like maybe with the mention of sister locations in the vintage tapes there are more incidents we're as of yet unaware of. I feel like the spring lock failures mentioned in the tape predate the death of Spring Trap and that the talks of sealing the rooms are in response to his death. I'm also convinced that Purple Guy is actually not the same guy in each instance, that there are "purple men"; some innocuous, some not. If the animatronics are consistently mistaking you for the killer who's to say they don't just see all staff as purple men?
Maybe the original spring failure was Golden Freddy (or Purple Freddy / shadow Bonnie) but when they attempted to clean his body from the suit he had to find something more malleable to take over, the puppet. And from that point on he's killing the kids out of resentment, blaming his death on them. (Or maybe if the spring lock suits are that sensitive it could genuinely be a kid's fault, if they hugged him or bumped into them maybe the released the mechanism.) But stuffing them into the other animatronics is just step one, he had to make them believe it was somebody else who murdered them to use them, to take revenge on the company. It's me is his calling card when he kills one of the new night guards, I'm still here, I've always been here and I'm the one doing this.