Jack was a regular teen. Not really, actually. He was underweight, an albino and very skinny. He had 3 friends, Hiccup, Rapunzel and Merida, him being the youngest.
Hiccup had invited all three of them to his party, as it was his last year at school. They all agreed and soon, they were all at Hiccup's house, enjoying some soda and pizza. Jack said he had to use the bathroom and Hiccup told him it was across the hall from them. Rapunzel said she wanted more soda, so Hiccup told her the kitchen was near the bathroom.
As Rapunzel walked by the bathroom, her new can of soda in hand, she met Jack, who came out of the bathroom. As they walked in the living room, they saw Merida and Hiccup sleeping. They walked by a window, Rapunzel shivering from the cold winds coming through it. Jack closed it as Rapunzel drank her soda. As he drank it, he choked a little as Rapunzel tried helping him. He excused himself to the restroom.
As Rapunzel sat worried, she heard a bloodcurdling scream from the bathroom. Hiccup and Merida woke up as they followed Rapunzel into the bathroom. They were scared stiff as they saw Jack at the bottom of the floor, bleeding. Pitch had heard the scream and had called the police.
Either Hiccup/Merida could've poisoned his soda, or Pitch had something to do with it. This time Rapunzel is somehow a suspect... I don't know.