Alpha Dog

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Johnny Truelove is a young marijuana dealer living in Southern California. His father, Sonny, supplies him with marijuana, which Johnny distributes to his gang of friends, including Jake Mazursky, who owes him a $1,200 drug debt. Jake makes a failed attempt at asking his father Butch and stepmother Olivia for the money. Meanwhile, Butch and Olivia are dealing with their rebellious teenage son Zack, who admires his older brother. A fight breaks out between Jake and Johnny when the former tries to pay Johnny only part of his debt. After back and forth retaliation, Johnny and his friends, Frankie and Tiko, go to confront Jake in person, but when they arrive at his house he is nowhere to be found. As they leave, they find Zack walking on the side of the road, and decide to kidnap him with the intent of holding on to him until Jake pays his debt.
Wanting a break from his home life, Zack makes no effort to escape. Johnny pawns Zack off on Frankie, who offers him a chance to leave, but Zack declines, not wanting to cause any problems for his brother. Zack stays with Frankie at his father's house and the two strike up an unlikely friendship. The next day, Zack ingratiates himself with Frankie's friends Keith Stratten, Susan Hartunian, and Julie Beckley, although Susan is the only one who seems concerned.
Frankie grows nervous when Johnny tells him that they could get in serious trouble for the kidnapping and hypothetically offers him $2,500 to kill Zack. Frankie furiously declines and Johnny claims it was a joke. Instead, he agrees to Frankie's plan to pay Zack to keep quiet. However, after a threatening phone call from Jake, and a phone call from his lawyer who tells him he could face life in prison for the kidnapping he decides the risk of ending up dead or in jail is too great to let Zack go. Johnny calls his associate Elvis Schmidt and offers to erase his debt if he kills Zack. Frankie and his friends still believe Zack will be returning home that night and throw a raucous going away party. Zack has a good time at the party, and later goes skinny dipping with Julie and her friend Alma, which leads to a threesome. After the party Julie gives Zack her number, and Alma gives him a goodbye kiss on the cheek.
Elvis arrives at the hotel where Zack is waiting to be picked up, and he and Frankie begin to argue when he reveals Johnny sent him there to kill Zack. When Frankie leaves Elvis takes Keith to dig a grave. Frankie offers Zack a final opportunity to escape, but believing that he is now a part of the group, and will be returning home soon Zack prefers to wait for Elvis to return. Meanwhile, Sonny, Cosmo (Johnny's godfather) and their lawyer confront Johnny, who refuses to call off the hit. When Elvis and Keith return to the hotel, Elvis and Frankie go outside to talk. Frankie, reluctant because of the friendship he has formed with Zack, finally relents when Elvis tells him they could face life in prison if Zack tells someone what happened.
Frankie, Elvis, Zack and Keith arrive at the grave site. Zack is not aware of what's going on but grows suspicious when a saddened Keith tells Frankie he can't go through with it and goes to wait in the car after giving Zack a goodbye hug. When Zack sees the grave, he begins to break down, and begs them to let him go. Frankie tells Elvis they shouldn't go through with it, but Elvis is keen on the job he's been given. Frankie calms Zack down, and ties him up with duct tape. Elvis then knocks Zack into the grave and shoots him multiple times with an automatic Mac-10, killing him. Zack's body is found three days later.
The epilogue shows the aftermath of the crime. Olivia, now suffering from obesity and depression, talks candidly about her failed suicide attempts and the loss she has experienced from her son's death. Susan angrily confronts Frankie over Zack's death and goes to the authorities. Elvis is caught while trying to secure a ride out of LA. Johnny flees the city and arrives at the house of old classmate Buzz Fescke, who drives him back to Cosmo's house, where he enters and is not seen again.
Tiko, Keith and Frankie are all arrested and serve their respective sentences - Tiko is convicted of kidnapping and sentenced to nine years in prison. Keith is convicted of second degree murder for digging Zack's grave and remains at the California Youth Authority until the age of 25. Frankie is convicted of aggravated kidnapping of special circumstances and receives a life sentence with the possibility of parole after 7 years. Elvis is convicted of kidnapping and first degree murder, He's currently on Death Row in San Quentin State Prison
The interviewer asks Sonny how his son was able to escape authorities for four years without help, but Sonny assures him he doesn't know where his son is. On screen text reveals that in 2005, after over five years of being on America's most wanted list, Johnny is finally found and arrested in Paraguay. Johnny is in California awaiting trial, and, if proven guilty, faces the death penalty.

Question: Why do Frankie and Elvis argue?
Answer: Elvis reveals to Frankie that Johnny sent him to kill Zack.