My theory on the story of the first murder. 1973:
Freddy and Friends are made as well as FredBear and Spring Bonnie.
Henry was the creator of the animatronics and William Afton was the technician.
Henry and William were also friends. They both had kids. William Afton had a 5 year old son named Michael as well as a infant son named (something) and a infant daughter named Elizabeth and Henry had a baby daughter named Charlotte. It was the good times.
But the good times do not last forever....
Soon after Mrs. Afton passed away. She was buried in the open clearing where her and William would always go to and lay down next to each other in and gaze at the sun at even before they got married and had Michael.
William Afton then got drunk after that as a way to forget his pain.
This caused him to become aggressive and irritable to Michael.
Eventually his sick dad or something moved in as well which gave him one more mouth to feed.
Let's just say, it was a lot.
And then William Afton with his broken family every day seeing Henry and his happy family was like a fucking insult and mockery to him. Henry wasn't intending it that way. But, like, seriously, imagine you lost your partner or somebody (all of you kids and teens such as myself who are not old enough to have kids just pretend you are old enough, just pretend, though, don't actually get pregnant in real life that shit sucks for any teen who becomes a parent), and then you see your best friend laughing and walking and talking with his family and kids that is completely intact... yeah you get the idea.
So then William Afton kills Henry's daughter and then drives home and then fights with Mike, and meanwhile puppet tried to save Charlotte but then the rain caused it to collapse and then it fell on top of her dying body, then Charlotte died, and then the puppet became haunted by Charlotte's spirit.