Here's the breakdown:
* Bob is killed by Ponyboy Curtis: In a moment of self-defense, Ponyboy stabs Bob after Bob attacks him and his friends, Dallas "Dally" Winston and Johnny Cade.
* Johnny's Role: Johnny is present during the attack and is the one who initially tells Ponyboy to run. He later helps Ponyboy hide after the incident.
* Johnny's Guilt: While Johnny didn't directly kill Bob, he feels immense guilt and responsibility for the incident, especially since he was the one who initially instigated the fight by getting into a scuffle with Bob's friend, Cherry Valance.
It's important to remember that Johnny's character arc is driven by this incident, and he ultimately makes the selfless decision to take the blame for Bob's death to protect Ponyboy.