would it be possibly for you to explain exactly what was going in your mind when you intuited that that yellow triangle guy possibly had a lot of cults, and was kind of culty and a cult leader before TBOB. Any canonical stuff we had back then which sent you in that direction?
Nothing but canonical stuff.
Bill's the object of mystical prophecies and apocalyptic warnings, marking him out as a figure of mystical/religious significance—exactly what cult leaders are TRYING to be marked as.
It fits his symbolic associations. he's the Eye of Providence! Mr. Illuminati! His very design is intended to evoke the idea of secretive conspiratorial groups, the occult, and the cultish.
And along with being a symbol people currently associate with conspiracy theories, the Eye of Providence was first and foremost intended to represent the eye of god. He's designed to look like a symbol of the divine.
In his canceled Weirdmageddon musical number, he sings "Trust in the all-seeing all-knowing eye!" "all-seeing eye," "all-knowing," and "in God we trust" are all Christian religious phrases about God and his omniscience; and Bill directly orders people to trust him like he's God.
He presents himself as a divine (or divinely powerful) figure multiple times. He tells Ford he's a "Muse"—a Greek god of inspiration. He introduces himself to Gravity Falls as "your new lord and master for all of eternity."
Before Ford's away of Bill's betrayal, he talks about info from Bill like it's divine revelation. ("The Muse has spoken!")
When he tries to talk Ford into switching sides, he describes his own goals as equivalent to becoming, and acting as, an omnipotent creator god: "Anything will be possible. I'll remake a fun world-- a better world!" "You'd be one of us: all-powerful."
We already know that he's gathered a gang of extremely loyal followers who treat him as their unquestioned leader: the Henchmaniacs. He leads them by using his charisma, threats of violence (from him) or doom (from Dimension Zero's decay), and the promise that he'll lead them to a utopia (Weirdmageddon).
Hell—he names his party "weird Armageddon." He himself names his goal like it's a religious doomsday. "Sure, it's the end of the world, but if you follow me, you'll get into heaven the VIP party! So the end of the world is a good thing! As long as you're following the right beliefs!" That's the argument of every doomsday cult in the world.
The way he lures people into working for him is identical to how cult leaders work (as well as con artists in general). I've written a long post about that here. A few highlights of things he does that parallel cult leader tactics:
he targets people who are socially isolated and emotionally compromised
he takes advantage of sleep deprivation to make sure his victims aren't thinking clearly
he regularly uses mystical manipulation
he socially isolates his victims from loved ones who could talk sense into them (Ford vs Fiddleford)
he regularly makes comments that boil down to "stick with me and do what I say and you'll get [insert your wildest hopes and dreams here], but if you don't do what I say you're doomed"
Let me elaborate on some of his demonstrations of mystical manipulation:
the mystical fires when he makes a pact
claiming to have secret knowledge; demonstrates this by showing off that he knows things about the characters and their lives they don't expect him to. (when Gideon asks how Bill knew his name, Bill says he knows LOTS OF THINGS; he would've looked less impressively all-knowing if he'd said "I just read your mind when I got here.")
claiming to have knowledge of the future (for example, confidently telling people when they'll die)
illusions of shapeshifting or magic tricks (and since most of the magic Bill does is in the mindscape, in his victims' dreams... they really are illusions)
Typically mystical manipulation means "pretending to have mystical powers," but in Bill's case he ACTUALLY has mystical powers yet he uses them like mystical manipulation: to impress people and make himself seem more powerful.
The way Bill uses his powers to impress victims is similar to the way Gideon uses his powers to impress his audience—and Gideon has so much in common with televangelists and religious con artists that I wrote an academic paper about it.
Bill presents himself as cool and smart and all-powerful and all-knowing; something cult leaders regularly do as a way to get power.
Also, desperately WANTING to be seen as cool and smart and all-powerful and all-knowing is a very common cult leader trait. Some cult leaders only want money and power, but some are motivated by a desperate need to be loved in the most toxic ways possible.
The way Bill surrounds himself with friends/followers who back up anything he says, and the way he tries so SO hard to recruit Ford by making himself seem likable—he reeks of being desperate for approval/admiration, but he tries to get admiration by putting himself in a position of authority over his admirers.
Also like many, many, MANY cult leaders: the closer you pay attention to him, the more clear it is that he's genuinely short-tempered, insecure, and is just trying to seem like he has it all figured out.
In the Bill Cipher AMA, he's asked what he thinks about his "fangirls," and he instructs his fangirls to take psychedelic drugs, walk into the Nevada desert, and assemble a human throne to await his arrival—in other words, "I want the people who highly regard me to put themselves in a compromised suggestible mental state and do a gruelingly unpleasant task to prove their devotion to me."
Arbitrary exhausting tasks + inducing mentally vulnerable states are common cult control tactics to prevent victims from thinking clearly enough to escape. Plus he's asking them to perform an extreme act to demonstrate submission to him as their leader/superior.
Cults are already a prominent theme in Gravity Falls. Example: the Society of the Blind Eye is the first and most obvious cult.
Example 2: Gideon's cultlike following, which frames him like a child preacher. This continues with his prison gang, which (metaphorically) worships him.
The show has other cult leader-adjacent characters: for example, Caryn Pines, lying phone psychic, who isn't a "leader" but who's manipulating and exploiting people's mystical/spiritual beliefs for her own gain.
Even Grunkle Stan: con artistry is just being a cult-leader without the religious claims or the long-term commitment. He's using charisma and lies to trick people into believing fantastical things. The themes that make up the foundation of cults are stuffed into every inch of Gravity Falls.
The big reveal that Ford knew Bill comes with revealing that he has dramatic-looking tapestries of Bill, a bunch of glass pyramids (and New Age religious movements LOOOOVE glass and crystal pyramids), and a statue of Bill that looks like the god Vishnu—strongly suggesting Ford might've been worshiping Bill.
Ford's meditating in a circle of candles to contact Bill also resembles New Age and witchy religious & magical rituals. It's likely that Bill himself guided Ford into worshiping him like that.
Is that enough canonical stuff?
Taken all together: Bill's an insecure guy who really wants popularity and power; he's a skilled manipulator and con artist; he's got all the tools needed to present himself as a mystical/divine figure; he's known to use those tools to recruit followers who slavishly adore him and do anything he says; and he's in a show where cults and secret societies regularly come up.
OF COURSE he's a cult leader. How could he NOT be?
