The Outsiders was raised elsewhere without the Greasers, he'd become Johnny the Homicidal Maniac. Hey, it's an Epileptic Tree, but still.
Darry orchestrated everything.Think about it. The Curtis kids are all smart in their own ways (Ponyboy is a literary genius, Soda is excellent with cars and women.) After Ponyboy ran away, Darry probably called up his old friends-turned-socs to help look for him and put the fear of God in him, teach him a lesson of some sort. Unfortunately, Johnny ruined his plans. That's why he had to die... OR Darry orchestrated even further: Bob faked his death, and when Ponyboy and Johnny went on the run, Johnny was feeding the gang info on their location. The fire was started on purpose (for some reason, DON'T QUESTION DARRY'S LOGIC), but Johnny was a casualty. Nevertheless, all is well in the end.