In case you're not aware "Cut Lex Luthor a Check" is a trope in which a character uses their technological genius for crime despite the incredibly obvious monetary gain that such technology could be used to acquire. Essentially, your villain is some sort of uber-genius who could legally market their tech and become a multi-billionaire, but instead they'd rather just use it to rob banks. Remember that Spider-Man comic panel where the dinosaur man is says "But I don't want to cure cancer - I want to turn people into dinosaurs"? That's CLLAC in action. I should also mention that CLLAC is not inherently bad and can be used well provided it doesn't veer too far into illogical absurdity.
William Afton takes it into illogical absurdity, but to be honest, I don't even care.
So fun fact about real life animatronics: they can move in extremely limited capacity and are overall fragile and require a lot of moving parts to create the illusion of a living creature. Afton clearly didn't get the memo because he was creating fully mobile bipedal (and bipedal movement is significantly less stable than quadrupedal movement) robots with enough speed to outright run, dexterity to climb through air vents, strength to easily kill grown men. Not only this but William, or possibly Fazbear Inc. given that he probably didn't want his Redditor-LARP getting discovered, somehow managed to also equip them with facial recognition processes. Oh, and in case you forgot, this all takes place in 1987. For some reason Fazbear Inc. is a subpar pizza joint and not a global tech conglomerate.
Later games add further notches to Afton's technological genius. Long before the days of the original Freddy's he was capable of designing animatronics that could double as costumes. Granted, they wre unsafe, sure, but still, it's a fucking iron man suit. He also possibly constructed an underground bunker underneath his house without anyone knowing it to house the Sister Location animatronics. Oh, right, the Sister Location animatronics! Not only are they fully capable of all the things the 1-2 animatronics are, they're also equipped to dispense ice cream, crawl on top of ceilings, and abduct children via giant metal claws. And this was before the events of the second game. In case you forgot, William used all of this technological genius for the sake of murdering children. He would've otherwise been a multi-billionaire known for revolutionizing the entire technological industry, but just decides that abducting kids and possessing robots with their spirits would be more fun.
Oh, and in case we forgot, yeah. Through his experiments, William discovered the physical existence of the soul. Not only that, he figured out how to essentially bind souls to physical objects like he's from Fullmetal Alchemist or some shit, essentially discovering immortality in the process. Forgot everything else, this would be inarguably the most important scientific discovery ever made in the history of mankind. William would be remembered as one of the most important people to ever walk the earth. Actually, he probably wouldn't even die because he could just use his alchemy powers to transfer his spirit to a robotic body. But nope, he just really wants to kill some goshdarn kids. Or resurrect his son or whatever theory-of-the-month is currently floating around the general community. You get the idea. Dude is not only an unparalleled tech wizard the likes of which the world has never seen, he also discovered the physical existence of metaphysical concepts.
In hindsight, the FNAF franchise probably jumped the shark by the time of Sister Location, and definitely cleared it with the release of the novels.