Complex sentence: When they go to his house he is nowhere to be found, but as they leave they find Zack walking on the sidewalk and kidnap him, with the intent of holding onto Zack until Jake pays his debt.
Split into simpler sentences: When they go to his house he is nowhere to be found, but as they leave they find Zack walking on the sidewalk. They kidnap Zack with the intent of holding onto him until Jake pays his debt.