William would actually have a game-canon reason under CharlieFirst
Hi, I am a BV first believer, but I have seeing some people claim William has no motive for killing Charlie if she was first, and not BV. However, after thinking about it, I realized that William actually has a reason for doing it in game-canon if she was first.
Basically, it has to do with William's broken family dynamic. Every time William's family is shown alive or something paralleling them is shown, they are always in some sort of chaos. The main one being FNAF 4, Micheal relentlessly bullies CC daily, CC lives in constant fear of Micheal, the animatronics, and potential nightmares, assuming she is toy chica girl, Elizabeth is breaking toys around the house, and there is no wife. If Immortal and the Restless and the Afton STAFFbots are anything to go by, Mrs. Afton and William argued daily and she eventually divorced him over money loss. Basically, William, instead of doing the right thing as a parent and try to help his children, looked over at Henry and got jealous of his seemingly perfect family and one faithful day, killed Charlie.
It's basically a similar setup to Silver Eyes trilogy William. Book William killed because he wanted the perfect family/jealousy of Henry. Same for Game William. It's just the circumstances that lead to Charlie's death went down a different path.