William Afton’s Secret Journals
This is evidence-based speculation, based on real facts, and not a head canon.
We know about the personal journals William kept, but not what was specifically in them.
What secrets did this mysterious journals hold? We are given a vague, one-liner:
TSE p. 351: A search of his house…found stacks of journals full of raving paranoia, passages about Henry that ranged from jealousy to near worship.
What was he paranoid about?
It could be the murders, however, police recovered them and if that were the case, they could have been used as evidence to convict him, unless he was very cryptic about it and used metaphors, perhaps. The only problem with this theory is that they were his personal journals, so why would he need to write in a way that would need deciphering, unless his paranoia was so extreme that he suspected the journals would be found.
Still, the descriptive word “raving” is used to emphasize bad and extreme qualities and is akin to gibberish, which would actually strengthen the cipher theory. It may have seen incoherent to someone who did not know the code. There is proof in the books that William was smart enough to use such a cipher. However, raving also means rambling, meaning lengthy and often changing topics.
Next we have Henry, one of the only specific topics that is mentioned. We already knew William was jealous of Henry from other points in the novels. Jealousy is a feeling of discontent or resentment caused by someone else’s possessions, qualities, or luck.
William definitely wants what belongs to Henry.
TFC p. 84: “…Henry found a unique spark-created something truly different, something he didn’t deserve, or intend, to stumble upon (Henry implanting his memories into a robot)…you will bring it (Charlie) to me.”
TTO p. 258: “I’ve been a master of all these creatures…All the animatronics…All of this is mine.”
TTO p.257: William to Charlie: “You have something that belongs to me.” (Henry’s memories).
One possibility is that William wanted to be a skilled as Henry at building animatronics. He seems to gloat and be prideful over his own animatronics and enjoys reinforcing the fact that he’s superior to Henry.
TTO p.258: “I’m…far more than Henry.”
TTO p. 261: “Do you think my robots would be as poorly designed as (Henry)?”
He must talk about it with Elizabeth, too, because she says herself,
TFC p. 99: “(William) was more brilliant than Henry.”
A possibly explanation is that William may have a college degree in Business and not robotics, like Henry. Henry was also a smart man and knew he needed to hire someone to handle the business end of his work. He would have most likely gone for someone with a degree in Business. This could mean that William may have not had any prior knowledge of robotics and engineering, unless is was an elective or hobby. Yet, he could have dual degrees, or maybe Henry chose someone else with a degree in robotics who was just knowledgeable in business.
TSE 350-351: Afton seemed like the normal one…the businessman. William Afton was the one who made Freddy’s a business, as he had (Fredbear’s Family Diner). Afton was as robust and lively as Henry was withdrawn and shadowy. He was a hefty man who had the ruddy geniality (friendly and cheerful manner) of a financially shrewd Santa Claus.
In either event, William borrowed a lot of what he knew from Henry.
TSE p. 344: “I helped (Henry) create.”
So Henry must have at some point allowed William to work with him and possibly taught him.
TFC p. 108: Jessica: “How did you create her?” (Circus Baby)
William: “I can’t take complete credit for this, unfortunately…My only real accomplishment was making something that could walk…No small accomplishment but it’s not happening as fluidly as you think. A lot of what you see is just in your head…that was Henry’s idea not to try and reinvent the wheel.”
The next word is worship. Since William knew Henry was not a deity, the next definition is honor given to someone in recognition of their merit (being particularly above average). Scott most likely meant that William looked up to Henry because he was so skilled at robotics. It’s highly unlikely it was romantic since he had no qualms about hurting Henry by killing his child.
So three things are clear: William looked up to Henry, but it caused him to envy him and eventually resent him.
Maybe someday we will get more insight into what else was in these mysterious journals.
OP does not claim this post to be canon, rather something to consider.