Final answer:
Johnny kills Bob Sheldon in self-defense during a violent confrontation, influenced by his past of abuse and the ongoing conflict between the Socs and the Greasers in 'The Outsiders'.
Step-by-step explanation:
In The Outsiders, Johnny's motive to kill Bob Sheldon stems from a need to defend himself and Ponyboy. The situation is dire as Johnny and Ponyboy are attacked by Bob and his group of Socs, who have a history of bullying the Greasers. Faced with the real threat of severe harm or even death, as the Soc's actions escalate beyond mere intimidation, Johnny acts in self-defense when he stabs and kills Bob Sheldon to prevent further harm to himself and Ponyboy.
Johnny's background of being neglected and abused shapes his perception of threats and contributes to his reaction. This act, while violent and drastic, showcases the underlying themes of class conflict, violence as a response to fear, and the harsh realities faced by youths in the novel. The Outsiders delves deeply into these complex emotions and motivations, which culminate in Johnny's drastic action against Bob Sheldon.