i think the barren world fits very well with the mood in this show. after all, whatever the Ruin is, if it's bad enough to destroy robots, you know that it must have caused equally destructive ends for all plant life. We had our first non-robot appearances last episode, and not only did the human not seem to be coping very well with the new world, but that lizard was almost so tough that you can't imagine anything but the hardest and toughest creatures survived...

also, they didn't destroy the moon (although, interestingly, there has yet to be a moon shown in the sky in a single episode) The person who Casshern killed was named Luna, and people are saying that because he killed her the Ruin started.