Though Don John ostensibly influences all of the action of the play, he has very few speaking lines for a main character. Don John operates as a plot-device more than a fully fleshed out character. He does give us a little speech about how he’s a bad guy—and likes being a bad guy—but there’s not much that we say about him because we never really know his motivations, or even his reaction to all of the chaos he’s caused.
In the end, he has run off before he can even be punished or have a warm, fuzzy change of heart scene. He’s definitely not Shakespeare’s most compelling and complex villain. Ultimately, though, it isn’t a failing of Shakespeare’s that this villain is so thin. It’s actually a reminder to the reader that the play isn’t supposed to be a tragedy, and isn’t even supposed to really analyze evil at all. The more important take-home points of the play are about hilarity and the folly of misunderstanding.