Maybe it's not the same as his twisted and sick interest in Ford but I think it'd be cool if people talked more about Bill's obsession with Stan. I just think it's interesting as well.
Ford is Bill's favorite (and in some contexts the object of his affections, for better or worse; mostly worse) while Stan is his most hated, the object of his afflictions.
These two didn't need to be introduced to one another or meet formally to dislike one another. They did so from just hearing or reading about eachother from the get go.
Bill thinks less of Stan thanks to Ford's comments about him; tagging him as a lower quality version of Ford that Bill later uses to hurt him with his attempted suicide call. Another tool for his plans that Ford was attached to. Bill saw the fight between the Stans take place and mocks his tattoo by giving It the meaning of "Watch your step" fully aware of the context on how Stan got it.
Even if it was unintentional, Stan's actions put a temporary stop on Bill's domination plans because Ford managed to escape him and now he had no one to activate the portal. Stan couldn't do much with just one quarter of Ford's notes and he couldn't fall so easily to Bill's manipulations like Ford did because Stan sees right through Bill as he's also a scammer. Stan has been in the business for just a decade at that point but Stan's targets were harder and tougher than the lonely down in their luck social rejects Bill preyed on. He knows honeyed words when he hears them.
In Bill's eye, this makes Stan not only dumb but useless. Infuriating to deal with.
The only use Bill finds for Stan is when Gideon gives him the job of stealing the code. To Bill, this is just a job with a high reward on the end. If he gets Gideon what he wants, Bill gets what he wants in the form of an apocalypse and potentially Ford's return.
Also once again, unintentionally Stan frustrates Bill's plans thanks to him repairing the damage Bill did to Dipper, showing him that he can do as he pleases in the mind and giving Dipper the key to defeat Bill. Heck, even the sight of Stan's body has Bill getting genuinely disgusted. He doesn't like him at all.
Even if the plan failed Stan managed to get the journals in the end and began to work on the portal right away. The date for the end of the world was only a matter of time for Bill.
I think it's safe to say that between Gideon Rises and Scary-Oke, Stan read the journal 3 and could potentially understand the reasons as to why his brother was so paranoid when they reunited after a decade. He states to Dipper that the town's weirdness is dangerous and if he could put two and two together that one of Gravity Falls's anomalies hunted Ford and drove him to insanity, it makes more sense why he wanted to protect the twins by hiding the truth. He lost one brother already, he's not losing the twins.
In the deleted scene of The Last Mabelcorn, Bill shows what he thinks about Stan by adressing him as Ford's dumb brother. Before that, Bill comments he was trying to prevent others from finding Ford, including Stan. Even if Bill doesn't think the highest of him, he's still on the look out for Stan. He did set the Weirdmaggedon plan back for long enough. The only useful thing Stan did was restarting the portal which created the rift. In the AMA, Bill sates that Stanley has done enough mistakes without his help.
The two don't interact during Weirdmaggedon until the finale. By then, Stan most surely connnected the dots of the Bill the guy who was working with Gideon in S1, the anomaly that made Ford lose his marbles and the demon causing the end of the world were the same person.
The apocalypse separated Stan from his family with Mabel trapped in the Bubble, Ford kidnapped by Bill, Dipper and his employees missing for a week or 4 days at best. He had to survive while having the doubt of wether the twins were alive, dead, or worse. The townsfolk and other magical creatures survived as best as they could but searching parties never found them. By the time they found Toby, Wendy and Dipper were long gone.
Stan doesn't like the guy responsible for the apocalypse but he's not stupid enough to dive into danger and risk the lives of his family but he goes with it in the end for majority of vote to save Ford.
(Possible foreshadowing here with Bill being in the objective and Stan on the other end?)
When the zodiac plan fails, Bill restrains not just Ford but Stan, mocking them both for their failure before capturing the twins. His eye is on Stan because he can tell this is mostly his fault.
To Bill, Stan is just another tool to ensure Ford does what Bill asks of him, like back during the Puppet Hour.
He restrains "Stan" once again when "Ford" agrees to the deal. He knows the guy would throw punches. Even if Bill is more powerful than Stan, he prefers to make sure.
It's in Bill's defeat where his annoyances with the human blossoms into hatred for Stanley. Stan went from a dumb annoyance with Ford's face to one of Bill's most hated foes.
Stan doesn't react to his mind getting destroyed which despairs Bill as his powers aren't working. Stan also makes sure that Bill knows his fatal mistake was messing with his family by saying it straight to his face with no fear, just anger and determination.
Bill desperately tries to wiggle his way out of his upcoming death by offering Stan everything he could possibly want but he doesn't take it. He tries to look dominating by glaring daggers at Stan but he cant hold his physical form for long and he melts and glitches out until Bill tries to reach out to Stanley to either help himself somehow or to take him down with him. (Really though, I LOVE how panicked and scared Bill sounds here. Peak voice acting there, Alex)
Bill's a wise guy but not all knowing. He knows lots of things but not everything. That includes the type of person Stan is. He doesn't take the offers because what Stan truly cared about was never money but family.
Stan punching him out of existance left Bill with an unremoveable scar and thanks to him dying by his hands, Bill is now stuck in jail and watching how Stan gets the happy ending Bill wanted: Recognition and admiration, Ford's forgiveness and company and last, but not least, a loving family.
Bill hates, HATES Stan's guts just possibly as much as, if not more than, Gideon did back in season 1 and tries to make him look bad by all means possible. He needs to humilate this pathetic copy of Sixer with an entire segment in a website. He needs to bring up the worst parts of his past to light, his fears, his failures, his deepest shames, darkest thoughts and lowest moments. Bill needs to ramble on how much of a loser Stan is. He needs to make it known that Stan didn't defeat him until he goes into a fit of rage. He needs to dedicate him a poem that is also a threath.
Bill thinks the least of Dipper and thus Bill's book gets two pages about how pathetic this kid is but Stanley is missing from most of the book until the end where he leaves a message for the reader. Right after Stan's part, Bill starts blaming Stan for his interference and ruining his plans with the reader and ends up cursing his name. Which is pretty funny considering Ford has been trying to contact the reader through the book far more times and Bill is aware of it, joking about it a bit in order to tease and mock Ford, but it's Stan appearing what sends Cipher into an anger attack. Heck, it was Stan's plan to erase his mind with Bill inside but he gives the credit to Ford.
The funny thing here however is that all he says about Stan in his section can be very much applicable to Bill himself. Both Stan and Bill are selfish but Bill lacked what Stan had; support. Or rather, connection. Love.
Bill was always alone from the very moment he came into existance whereas Stan was Ford's partner in crime, his family. They had eachother while Bill only had himself.
Both of the worst mistakes of their lives were accidents but Stan grew enough to realize his own failures and spent thirty years trying to fix them while Bill kept blaming everyone but himself, refusing to acknowledge his own shortcomings. He took his anger and hurt out on the world and never grew up because it meant confronting what he did. The deaths of his family and home.
Stan shows Bill what he could have gotten if he had done it. And that's why Bill hates Stan so, so, so, so, so, so much.
Bill may think Ford is his other half but Stan is his literal mirror. Bill just hates his own reflection but isn't that truly aware or emotionally mature to realize it.
After all, it's easier to blame everything on someone else. Anything is better than confronting his own troubles. And this is why Bill is doomed. He's insane and you know how one definition of insanity goes: Insanity is doing the same thing over and over but expecting different results.