why does claudio send don pedro as his emissary to hero to declare his love? why does don john want to cause trouble? why is he so morose?

I don't think that Claudio sends him. Benedick tells Don Pedro of Claudio's crush on Hero and that Claudio is too shy to profess his love. Don Pedro decides to act on Claudio's behalf disguising himself as Claudio and wooing her at the ball. After he will inform Hero's father and all will be well. Don John is angry that the rebellion he waged against his half-brother Don Pedro failed. He basically wants revenge. He is morose because his brother is getting all the power and praise . He feels jilted and now humiliated that Don Pedro has forgiven him.