I think the world has mostly ended because the cities we wander through are as rotten as we are. Buildings have collapsed. Rusted cars clog the streets. Most glass is shattered and the wind drifting through the hollow high-rises moans like an animal left to die. I don't know what happened. Disease? War? Social collapse? Or was it just us? The Dead replacing the Living? I guess it's not so important. Once you're arrived at the end of the world, it hardly matters which road you took.