Why does Don John tell Claudio that Hero was being disloyal?
Don John is the prince Don Pedro’s illegitimate brother. The two have only recently become reconciled, but Don John still deeply resents Don Pedro. He feels trapped: “I had rather be a canker in a hedge than a rose in his grace … I am trusted with a muzzle and enfranchised with a clog.” Don John is so eager to spite his brother and his friends, he latches onto Borachio’s news of an impending marriage between Hero and Claudio. He expresses jealousy of Claudio’s esteemed position with the prince.
Don John and Borachio trick Claudio into thinking that Don Pedro wants Hero for himself. Don John seems to genuinely believe that his brother is that selfish and duplicitous: “Sure my brother is amorous on Hero and hath withdrawn her father to break with him about it.” Claudio briefly falls for this plot, but the deception is soon cleared up.
However, Don John and Borachio do not give up. Don John says, “Only to despite them, I will endeavour any thing.” He pays Borachio to help him. Borachio plans to have a liaison with Margaret, Hero’s “waiting gentlewoman,” and pretend that she is Hero. They will appear in Hero’s room, she will dress in Hero’s clothes, and he will call her Hero. Don John leads Don Pedro and Claudio to Hero’s window, “proving” her disloyalty. The three of them then publicly shame her at the wedding.
Essentially, Don John plans to ruin Hero in order to get at Don Pedro. It is a form of revenge that includes innocent casualties.