Mean Creek

When small and shy Sam admits to his older brother Rocky that the school bully, a dyslexic boy named George, has hurt him because he moved George's video camera while George was filming himself playing basketball, Rocky devises a plan to exact revenge on George.
Rocky recruits his friends, Clyde and Marty, to assist him with his plan. Part of the prank entails taking George on a boating trip to celebrate Sam's birthday party. The ultimate joke, in their opinion, will be when they get him to strip in a game of truth or dare, then make him run home naked.
Sam invites his girlfriend Millie along. He does not tell her the plan until they arrive near the river. Millie refuses to continue until Sam promises that he will call the plan off, which Sam agrees to do. Sam tells his brother to stop, and Rocky tells his friends what Sam has conveyed to him. Although Clyde has no problem with it, Marty is very reluctant to not go through with the plan. Throughout the trip, George attempts clumsily to fit in with the others by telling rude jokes, which the other members of the group do not find amusing. The group soon realizes that although George is annoying, he is very lonely and just wants to be accepted.
On the boat, Marty deviates from the others' plan and initiates a game of truth or dare, though the rest decide to go along. After George shoots Marty with a water gun in good fun, George makes a funny quip about Marty's father, not remembering that it is a sore subject as Marty's father killed himself. This sets off Marty, who tells George the whole plan and starts to ridicule him.
Angered and humiliated, George launches into a vulgar tirade against everyone else on the boat, ending by crudely mocking Marty's dead father. Marty snaps and Rocky, in an attempt to stop the fight, accidentally pushes George off the boat. Unable to swim, George struggles to remain afloat in the water. As the others regard the scene in horror, George accidentally hits his head with his video camera and does not come to the surface. Rocky dives into the water but is unable to find George. Minutes later, George appears face down in the shallow water close to the shore. Rocky exhorts the others to help him bring George to shore, where Millie gives him CPR. The effort is in vain as George is dead and it is apparent that he cannot be revived.
The group is traumatized and in fear of being charged with murder. They dig a hole and bury George. Clyde's plan is to explain that it was an accident but Marty threatens them, gaining the complicity of both Clyde and the rest of the group. As they had already tricked George into not telling his mother where he was going, she would not know of their involvement. Marty speaks to the only witnesses of George with the group, his brother and his brother's friend, and they agree to keep quiet.
Marty goes to tell the news to his friends, who have all gathered at Sam and Rocky's house. They are willing to accept the consequences as opposed to having the guilt of George's death hanging over their heads. Marty refuses to turn himself in and feels betrayed by all of them. He storms out and convinces his brother to give him his gun and car. The brother again agrees to the favor, albeit reluctantly. Marty robs a gas station with the gun and drives off, becoming a fugitive. Meanwhile, the others go to George's house and confess to his mother.
Sam is later seen in a confession room, telling the story to the police, who later find and view the tape from George's video camera. In a final scene, audio of George explaining his dream of becoming a filmmaker and documenting his life in hopes that those who see it will finally understand him plays in the background while Sam watches the sheriff exhume George's body.

Question: What causes George to launch into a vulgar tirade ?
Answer: Anger and humiliation.