Don John is a bastard, in both the legal and ethical senses of the term. The essence of his character is that he is evil.
Having recently rebelled against his half-brother Don Pedro and lost, he is forced to follow the winners to Leonato’s. He does not admit having been in any way wrong. His general disposition is melancholy, that is, depressed and depressing. He believes that the more depressed he appears, the happier his brother’s followers are. However, he fully intends to work any mischief he can, and as Claudio was one of the chief instruments of Don John’s defeat, the thought of ruining his life is particularly pleasing. While his first attempt to spark jealousy in Claudio is only momentarily successful, his follower Borachio’s plan works marvellously—so well that Don John pays him a thousand ducats in recompense. Having accomplished his desired evil ends—Hero dead, Claudio distraught, Don Pedro shamed, Leonato dishonored—Don John flees, but is caught. It seems that Benedick will arrange his punishment, a thought likely to strike fear in him.