Stanford Pines was going to ruin everything.
Bill had worked to secure this universe for himself long ago, even before the arrival of its citizens. He had woven himself into the threads of history, hiding in every culture. He was the giver of knowledge, the bringer of light and the muse of artists and inventors. Humans were his pets and his toys, and he did as he pleased with them. He built civilisations, constructed religions, and burned empires.
A few bright humans suspected a manipulator. The ones who spoke about him were shunned and shut out from society until they were completely alone. Those were the most fun to play with. Humans with the mental capability to discover him were a rare toy that didn't show up often. Stan was once one of these special toys, before he pushed Bill away.
Despite all his intelligence, Stan still didn't realize that it was impossible to go against Bill's wishes. He could try as hard as he wanted, gather all of the journals, and even start the portal, but, no matter how hard he tried, he would not leave the universe that Bill had worked so hard to cultivate. Jumping universes would make the hologram wiggle and blur. The illusion would be ruined, the board would be flipped, and the game would be over.
He couldn't allow that.
But Stan's unwelcome interference presented Bill with a wonderful opportunity. He had finally had a chance to claim the Pine twins as his own. They would be useful to him, a pair of allies to aid him in his work.
The twins were ideal for this situation. They were close to Stan, so the chance that he would harm them was minimal. They were naive enough not to question what he told them too much, but competent enough to finish the job.
Bill had watched Dipper attentively since the "battle". ( It wasn't so much of a fight as it was a test of valor.) He had seen everything. He watched as Stan lost the Mystery Shack and Dipper fought to get it back with a gnome army (which, Bill had to admit, was creative). He saw Dipper jump off a cliff to save his sister.
The boy was brave, resourceful, and clever, but he was also fickle, impulsive, easily influenced by his emotions, and prone to bouts of hopelessness. He was both powerful and easily manipulated, perfectly suited to Bill's needs.
Mabel, on the other hand, was almost perpetually bright and optimistic. Assertive, creative, gutsy, caring, and silly, she was the counterbalance to Dipper's serious nature. Dipper in turn tempered her silliness, keeping her from enacting the more dangerous of her ideas. Without each other, the twins would be unbalanced. They worked together as easily as two sides of the same brain.
Now all he had to do was get them to work with him.