Jake Mazursky: I will take you down to hell with ME.
Olivia Mazursky: They said there's a reason for everything. Well if God's got a purpose for me, he better get the fuck down and show me, cause I don't see it.
Frankie Ballenbacher: You want a cigarette?
Zack Mazursky: No, I don't smoke.
Frankie Ballenbacher: Fuck that. It's good for you.
Alma: Have you ever been laid, Stolen Boy?
Sonny Truelove: You've fucked up your life so bad! And the only way you're gonna unfuck it, is that you come clean now, Johnny, you come clean NOW.
Frankie Ballenbacher: Where is that motherfucker?
Johnny Truelove: Who the fuck knows? I don't know! He's probably lying low waiting to shoot me in the fucking head! I mean, think about it, that's what I'd be doing.
Frankie Ballenbacher: Stop already, all right? You know how I get with this anxiety shit.
Olivia Mazursky: My son was murdered over $1200.
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