Trivia: At Frankie's house, when some girls visit during the daytime, Frankie turns to Zack and calls him 'Yankele'. Yankele is a Yiddish [German-Hebrew] affectionate version for Yaakov, or Jacob, since Zack is Jewish.
Trivia: Jesse James Hollywood, played by Emile Hirsch, the mastermind behind the kidnapping of the young boy, was recently found guilty on July, 2009, of the crime depicted in this film. He was sentenced to life in prison.
Trivia: During the fight scene between Sharon Stone and Ben Foster, Stone hit Foster so hard in the face that caused his nose to bleed. He told her to do this as he wanted to make the scene as realistic as possible.
Answer: If you're talking about why they kidnapped Zack, Zack is the younger brother of Jake. Jake still owed Johnny money, but when Johnny and his gang go look for Jake, they can't find him. They see Zack and kidnap him, essentially for ransom, until Jake fully pays Johnny back.
Bishop73
It's also important to remember that this is a film based on true events, the kidnapping and murder of Nicolas Markowitz. All the names in the film have been changed.
Why did they murder Nick?
Although this should be posted as a different question, in real life, Jesse (the one Nick's brother owed money to) had decided to let Nick go home. However, Jesse found out what kind of legal consequences he'd face for kidnapping Nick and decided to have Nick killed instead (obviously thinking he could get away with murder and deny any kidnapping allegations).
Bishop73