I actually enjoyed Mad Max. Here's a theory someone posted on reddit about the movie. Kinda makes sense. I'll link to it since it has spoilers.
http://www.reddit.com/r/movies/comments/368fkl/fury_road_theory_who_killed_the_world_four/
It's more simple than that to me.
Immortal Joe is satire on the military industrial complex and religion in 2015.
He has books, a piano, and his living quarters is artistic. He's a very educated man, especially since he has an irrigation system, milk farm, and greenhouse. Only a few in his inner circle know about this. He's the only one that can open his vault and go in there. He's prevented all of his underlings and warboys from reading boys and being educated. He gives them and the people outside just enough to where they worship him, but they don't try to kill him.
As we saw in the movie, Joe controls the Gas Town and Bullet Farm as well. In the beginning, he gives a big speech to the people about Furiosa and the War Boys to go off and fight to get Gas in Gas Town but it ends up being that they're all colluding together. So Joe creates fake wars to get the approval of the angry mob.
The War Boys' life dream is to go to the afterlife (The Gates of Valhalla). The spray paint is sort of like a high, or a symbol of martyrdom. When the first guy uses it and then proceeds to sacrifice himself, the other warboys say "WITNESS!!". They witnessed an act of their comrade going to the gates of valhalla. That's also why Nux was so mad because he had 3 opportunities. Red Haired girl convinces him that it was all a religious sham.
All of Joe's breeders know Joe is just manipulating the populace. The "Who Killed The World?" thing isn't necessarily who literally is the reason the world came to ****, it's more of "who killed hope for rebuilding civilization?" And it's Immortal Joe as he controls everything from guns, food, water, gas, war, etc.