Factual error: Several characters can be seen playing an Xbox game console, which was not released until 2001, although this movie is set in 1999.
Factual error: Throughout the film, whenever officers from the Los Angeles Police Department are depicted they are wearing a fake police patch. This is obviously incorrect because officers with the Los Angeles Police Department do not wear patches.
Factual error: When Frankie Ballenbacher arrives at Johnny's house when he wants to go to Fiesta, he is wearing some brown and white Nike Air Force I's that were not released until 2004. The movie is supposed to take place in 1999.
Factual error: The Land Rover discovery the girls are driving, looking at the headlights is an 03-04, the movie is set in 1999.
Factual error: When the police cars are in front of the kidnapped kid's house, it shows the date as Sunday November 8th, 1999, which is incorrect. November 8th, 1999 was on a Monday.
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