In this comedy Shakespeare creates a very distinct and obvious villain: Don John, the bastard half-brother to Don Pedro. Don John is a two- dimensional figure dominated primarily by melancholy. He is always gloomy and sad. And because of this characteristic, he wants to create trouble for his brother. He wants to create chaos.
Shakespeare does not go to any length to explain the motivation for Don John. As an illegitimate son of a ruler, he would be unable to inherit any title. He could never become a prince. So, perhaps, this makes him bitter. Also, during the Renaissance, people in England believed that all bastards -- all illegitimate children -- were somehow mentally and morally corrupt or inferior. They believed that something was wrong with them and that they were not the same as people born from legitimate (or married) parents. Shakespeare relies on this belief or superstition in order to avoid any lengthy and unnecessary explanation regarding his villain.
Shakespeare also relies on this belief regarding bastards in his tragedy of King Lear. In that play the bastard Edmund betrays his own father, the Earl of Gloucester. Yet Shakespeare then reveals that the fact that he is a bastard is not really the true cause of motivation; for even the legitimate daughters of King Lear, Goneril and Regan, also betray their father. Both legitimate and illegitimate children, then, can be morally corrupt and even evil.
Don John is, as he admits, a "plain-dealing villain" (line 25). He does not hide the fact that he is unhappy and sullen. He compares himself to a chained dog that needs to be controlled: "If I had my mouth, I would bite" (27-28). He has a vicious nature, and just wants to lash out against others, especially his brother. Don John, then, becomes a symbol of wildness and emotion. He does not act rationally, and he does not care to do so. He is then, quite simply, the two-dimensional villain of the play.
The second instance of eavesdropping also is noted in this scene. Borachio, one of Don John's men, also overheard Don Pedro talking to Claudio. But unlike Antonio's not-so-sharp servant, Borachio overheard the conversation correctly. Borachio tells Don John that Don Pedro intends to help Claudio win Hero as his wife. But if Don Pedro wants to help Claudio, then Don John wants to hurt Claudio. Don John wants to subvert anything that his brother wants. So, he starts to think of a way so that he can stop Don Pedro from helping Claudio.
The reader should note that this is a case of double-eavesdropping. The good characters (Leonato and Antonio) receive the wrong information, which leads to misunderstanding and trouble. But the bad characters (Don John and Borachio) hear the conversation correctly, and this leads to trouble and mischief. In this situation, then, eavesdropping is negative on both sides. Yet eavesdropping can also lead to positive results, as Shakespeare reveals later in the play.