Mad Max/WMG

Johnny is perfectly innocent.
We never see him perform any of his crimes, he's only left there at the end of the rape scene. He's thought of as a wimp by the rest of the gang. He visibly refuses to burn Goose alive. And, of course, he may have just found the truck destroyed and needed shoes.

Lord Humungus is actually Fifi
Think about it. He wants to bring heroes back to Austrailia. What better way to do it than be the Villain.
 * Plus, Fifi was a pretty big guy, as was Humungus.

Mad Max is set in the Fallout universe.
Australia was physically mostly untouched by the bombs but society broke down just like everywhere else.