Death Wish 4: The Crackdown

Erica Sheldon (Dana Barron), the teenage daughter of Karen Sheldon (Kay Lenz), Paul Kersey's current girlfriend, goes with boyfriend Randy Viscovich (Jesse Dabson) to an arcade to meet up with a man named JoJo Ross (HÃ©ctor Mercado) and another buddy, Jesse Winters (Tim Russ). JoJo offers her crack cocaine, and Erica dies from an overdose. Having seen Erica accept a cigarette from Randy while in his car the previous night, Paul suspects Randy was involved with Erica's death, so he follows him to the arcade. Randy confronts JoJo and threatens to go to the police. JoJo murders Randy to prevent this. Paul promptly shoots JoJo, who falls onto the electrified roof of the bumper-car ride to his death.
At home, Paul receives a call from secretive tabloid publisher Nathan White (John P. Ryan). Nathan says that because his daughter became addicted to drugs and eventually died of an overdose, he wants to hire Paul to wipe out the drug trade in LA. There are 2 major gangs competing for the local drug supply: one led by Ed Zacharias (Perry Lopez), the other by brothers Jack (Mike Moroff) and Tony Romero (Dan Ferro). Kersey accepts and Nathan supplies him with weapons and information. LA detectives Sid Reiner (George Dickerson) and Phil Nozaki (Soon-Tek Oh) investigate the arcade deaths.
Paul infiltrates Zacharias's manor. As he bugs a phone, he witnesses Zacharias murder a colleague who stole a big deal of cocaine from the cartel's South American connection. Zacharias discovers and captures Paul and orders him to help carry out the dead body. A hired hitman, Al Arroyo helps Paul hide the corpse in the trunk of a car. Paul kills Arroyo with the car's trunk cover in self-defense.
Paul proceeds to kill three of Ed Zacharias's favored hitmen at a restaurant with a bomb concealed in a wine bottle. He kills drug dealer Max Green (Tom Everett), leader of Romeros' streetdealers, disguised as a sex video trader. He confronts the Romeros's top hitman Frank Bauggs (David Wolos-Fonteno) in order to find out more about their cartel, but a fight ensues and Bauggs falls off his apartment to his death. A few days later, Nathan instructs Paul to go to San Pedro, Los Angeles, where a local fisherman wharf acts as a front for Zacharias's drug operations. Breaking in, Paul kills eight more criminals and blows up the drug processing room with a bomb. Detective Nozaki reveals himself to be a representative working for Zacharias, and demands that Paul tell him who he works for. Paul refuses and kills him. He lures Zacharias and the Romero brothers into a trap, leading to a shootout in an oil field in which both cartels are completely destroyed. Paul personally kills Zacharias with a high-powered rifle. Nathan congratulates Paul, but sets him up with a car bomb. Enraged, Paul returns to the White Manor only to find a stranger who claims to be the real Nathan White; the impersonator who hired Paul was actually a third drug lord who used him to dispose of the rival cartels. Paul is approached by two cops who arrest him, but he recognizes them as fakes, causes their car to flip over, and escapes.
To get rid of Paul, the Nathan White impersonator kidnaps and uses Karen as a bait. Detective Reiner waits inside Paul's apartment to arrest him for Nozaki's murder, but Paul knocks him out. He arms himself and goes to the meeting place designated by the drug lord: the parking lot of White's commercial building. The car rolls forward and the drug dealers spray the car with bullets before realizing Paul's not in it. Paul fires a grenade, destroying a van full of bandits, then fires another to kill Jesse as he betrays his crew and tries to drive away. Paul follows the drug lord into a roller rink, but he escapes through a back door, holding Karen hostage. Karen attempts to escape, but the drug lord shoots from behind and kills her. Distraught by Karen's death, Paul fires a last grenade that finishes him off. Reiner arrives and orders him to surrender, threatening to shoot as Paul walks away. Paul replies, "Do whatever you have to", and Reiner lets him go.

Question: why does Nathan want to hire Paul?
Answer: To wipe out the drug trade in LA.