In case anyone is wondering why the demon king got serious when Rudeus said he scratched the dragon god.
The dragon god can only be hurt by (I believe) king level things (third best with the best being God level which can change the world) and just this fact alone makes Rudy a incredibly powerful individual
Ps. why rudeus is acting a bit wimpy is because he doesn’t actually see himself as strong just stronger than normal (feeling the power of the top (dragon god) and being around absolutely amazing individuals tends to do that to you)