Answer:
Johnny leaves on fear because of the abuse he suffered from his father and the Socs. However, fearing his and Ponyboy´s dumped cigarette butts could have set the church on fire, he surprisingly takes responsibility for the kids' safety and runs to their rescue.
Step-by-step explanation:
Furthermore, he saves Ponyboy's life pushing him outside and gets badly hurt by the falling roof, leaving him in a critical condition that leads to his death. He goes from being considered a misfit to be a hero.
Before he dies, Johnny realices that his innocence is lost, but finds comfort on having save the kids from physical injury and from loosing the innocence he had long lost.