Well, the main "baddie" or villain of the play is definitely Don John, the bastard brother of Don Pedro, who at the beginning of the play has just had his rebellion against his brother's rule smashed and has been welcomed back into his brother's court. However, it is clear from Act I scene 3, when we first see Don John out of the public gaze with his two henchmen, Conrad and Borachio, that he is anything but resolved to his defeat. Note what he says:
In this, though I cannot be said to be a flattering honest man, it must not be denied but I am a plain-delaing villain.
It is clear that he is incredibly displeased by his defeat and his own low social standing compared to that of his brother's, and thus hatches a plan to attack one of his brother's most trusted friends and slander his bride. Note what he says about the idea of attacking Claudio is such a subtle way:
This may prove food to my displeasure. That young start-up hath all the glory of my overthrow. If I can cross him in any way, I bless myself every way.
Thus the stage is set for the deception of Claudio and Don Pedro and the slander of Hero, engineered by Don John, the definite villain of the piece, ably assisted by Borachio and Conrad.