By his very position in Don Pedro's household, Don John recognizes that he is a "second-class citizen." When he is unsuccessful at gaining recognition through military battle with his brother, he turns his discontent toward conspiracies to hurt those he envies — his brother and Claudio, for example. When Claudio's wedding is disrupted, he runs away, leaving his coconspirator behind unpaid — perhaps a fatal error. Might he have felt any remorse at hearing that Hero died of her shame? Probably not. Anyway, he has been caught and will return for his punishment, at least partly shaped by Benedick.
Is Don John a changed person? Probably not, but then we wouldn't expect true villains to change, especially in a comedy.