you forgot something important in the Afton family. so Wiliam was in Springtrap during the events of FNaF 3, right? yea. Michael felt horrible for what he did to Cassie/The Crying Child/FNaF 4 Protagonist, his brother, and went to face his dad. and then, after the events of FNaF 3 and he burned down the building... went to Circus Baby's Entertainment and Rental, which is under the FNaF 4/Afton house. Down there, he wished to save his sister, to make things right, who was killed and stuffed into Circus Baby not too long after the opening of Circus Baby's Pizza World. he tried to save her and stuff, but Wiliam had it as a trap. He knew what would happen. The animatronics used the scooper on their selves, became Ennard, then used it on Michael, and got inside his skin. They wore him for a long time before leaving. He should have died, but never did. He decided it was time to give his paps a little visit. He eventually, after the events of FNaF 6/Pizzaria Simulator (ill talk about something important there soon), dragged himself and his father down to hell. not real hell, but pretty much. Ultimate Custom Night, which was in replacement of the FNaF 6 location, Freddy's Pizza Place. we play as William himself. The One Who You Should Have Not killed is Michael Afton.
Wiliam eventually found a way to get out after a long while and finally passed through his eternal suffering, he, as Burntrap after the building burned long ago, managed to contain his body in a pod, where he created a digital version of himself, known as Glitchtrap. He used it to try to take vital information about remnant to continue his experiments, until eventually being shut down. (i'll get into the depths of Jeremy and and all of dat some other time)
(Henry burned down the FNaF 6 building, building it to burn the animatronics and relive the nightmares he's had. he didn't do anything earlier, because he was in jail. he was punished for the crimes, because he was the one who designed and built the suits.)
(Also, William killed them all for remnant, the good memories. there wasn't many deaths. also Puppet/Charlie helped the missing kids realize who they are and what happened.)
(WIlliam broke them in the FNaF 3 minigames, which happened in the FNaF 1 building, to get the remnant to make more experiments to try to bring back his son, Cassie/Crying Child/FNaF 4 Protagonist. he did everything he did to try to save his son's life, so that's some VERY important news. he killed to try to bring...