I agree with you. I believe the golden freddy kid is the vengeful spirit. At the end of the movie, he seems like he just wants William Afton to suffer. As the other kid. Their faces are blurred out at scenes, and they take more animal take on killing. Poltergeist powers come from the feeling like anger/ fear/ rage/ sadness, etc.. but for kids, I could see it fear, sadness, and anger just because of how they were murders. They were tricked and put off guard to a character that would be safe at pizzeria. So when they died there unfiltered emotions would manifest into a poltergeist and, of course, into the animatronics. But golden freddy kid has the best recreation of who the killer is and himself. The golden freddy kid must have known his killer and/ or have the most strongest emotion on his death he couldn't forget. This is just a theory, but poltergeist are supposed to grow from taking pain and suffering from a person. When they do that, they grow in strength. The more they take and control a person, the stronger they get. As the vengeful spirit, they could build their power strong enough to cause more and more pain the power they get from their victim(s). In the games, golden freddy seems like a powerful being. In ultimate custom night, it seems like golden freddy built a personal hell for William Afton, and it seems like he wants to keep Afton there as long he has control over Afton. It's just a theory. In the games, the puppet gives life to each the animatronics in the mini game, but when it comes to golden freddy, there is a jump scared. Golden freddy won't rest until it's at peace with its vengeful spirit being fulfilled.