As soon as he sent Jeannie away he went to the globe, he was saddened by what he had to do. With Jack working with the Guardians, even though he said he wasn't, he couldn't risk Jeannie getting hurt. He had hoped that seeing the lights turn dark would make his mood rise but instead the lights looked brighter than ever! "The lights? Why aren't they going out?" Kōri whinnied at him, "They're collecting the teeth?" The fairies, what Tragedy had thought were humming birds, started tweeting joyfully and buzzing around the cages at Pitch's exclamation. "Pipe down or I'll stuff a pillow with you!" he shouted in frustration. He even growled at Kōri, making her wish she hadn't been so cruel to Tragedy. She sank to sand and tried to hide in the shadows like her master did when she was upset. Pitch got a dark look on his face as he spun some of Kōri's sand in his hand, Jeannie was gone, and he could do whatever he needed to get rid of the guardians! "Fine. Have your last hurrah. For tomorrow, all your pathetic scrambling will be for nothing," he created a small Sandman and crushed it in his palm. He would find a way to kill Sandy and his Nightmares could take over the dream world, "there is more than one way to make a light go out," he chuckled yelling for Kōri to gather the others and go fetch the Guardians. He then looked at his palm where Kōri's sand had been and he remembered Jeannie. Had he been too harsh? No, she was safer the farther she was from him and those goody-two-shoes Guardians. Pitch still had pain in his heart as he sat up his plans, to trap Sandy.
When Pitch heard they were in the old James house he laughed at the irony. He told the Nightmares to bring him Sandy. They tried but Jack followed and when Pitch saw Jack he forced himself to contain his emotions. He asked Jack why he kept butting in and when he mentioned the teeth Pitch was confused. Why would he worry about the teeth when there was the case of his little sister being stolen away? Pitch turned away from Jack when Sandy snuck up behind him; he stumbled away but fought with slight purposeful weakness. When he fell from the roof he had the Nightmares fight Sandy and Jack while he jumped on Kōri's back. The other Guardians arrived but were distracted by all of the Nightmares Pitch had. Pitch rode Kōri's sand up to where Sandy fought a group of circling Nightmares and he turned some of her sand into an arrow of fear, aimed it at the mute Guardian, and let it go into his back. "Don't fight the fear, little man!" Pitch laughed. "I'd say 'sweet dreams,' but there aren't any left."
Sandy crumbled and all of his gold sand turned a beautiful black. His last look, though, was a look of true strength; he held his head high and his mouth taut as he closed his eyes and let the pain leave him.
Tragedy would have been proud at the way Sandy went, Pitch thought, then he forced himself to focus on his sand because Jack was charging him. Pitch sent all of his Nightmares formed from Sandy's demise to fight Jack but he iced all of Pitch's Nightmares using a force from his body strong enough to knock him out. As Pitch fell from his broken platform his only thought was of what he had done, "Kōri?" he questioned. She was Jeannie's and he had used her for his agenda now Kōri was gone.
As he hit the ground he saw some Nightmares had escaped the blast one being his fastest and strongest, Kōri. His focus and mood immediately brightened and he laughed, "finally, someone who knows how to have a little fun!" Maybe Jack was powerful enough to help him; maybe he could use him and strengthen his relationship with Jeannie. Pitch decided to prepare to help Jack and to find out what was the deal with the teeth. Why would Jack care about the teeth? His memories! Jack doesn't remember Jeannie or his past! Giving him his teeth would get him away from the Guardian's and give Jeannie a surprise when she returned! If she returned.
I hope you enjoy this side story for Pitch! (Since Jeannie/Tragedy is not in the movie at this point). It is from the movie just explained in my own way.