One of the enduring questions about Much Ado About Nothing is why Don John, the Prince's illegitimate brother, goes to such lengths to destroy Hero's reputation and therefore her marriage to Claudio, one of Don Pedro's closest companions. In other words, what has Hero done to deserve such treatment? The answer is not a thing, but that is beside the point.
In Act II, Scene II, Borachio, Don John's supporter (actually, as it turns out, henchman), explains a plot that will have devastating results—the ruining of Hero's reputation, the complete embarrassment of Don Pedro, Claudio's love for Hero destroyed, and even perhaps the death of Hero's father, Leonato, presumably from shock.
It is bad enough that Borachio could create such chaos, but it is even worse that Don John—even though he is no friend to his own brother—would consent to the destruction of an innocent. His reaction to Borachio's plan, of course, leads to an insight into his villainy:
"Only to despite them I will endeavor anything."
Don John's primary target, as Professor Emma Smith has observed, cannot be Hero—even though she will suffer the most immediate harm. Don John's goal is to destroy the two primary men involved: his brother and Claudio. In this plot, Hero is what we now euphemistically call "collateral damage"; that is, she is the person whose disgrace will, in turn, devastate both Claudio, for having loved her, and Don Pedro, for having wooed her on Claudio's behalf. Don John essentially seeks to cast doubt on Claudio's and Don Pedro's ability to discern true worth and, by extension, their ability to judge all other things rightly.
In a world in which rulers must appear completely dependable and accurate in their judgment of people and motives, Claudio's and Don Pedro's failure to discern Hero's flawed character renders their ability to judge all other things suspect, thereby casting doubt on their inherent right to rule in a turbulent world.