Angelo is the bad guy in this play, mostly because he tries to get in the way of Isabella's life-long dream to become a nun. Angelo is also an antagonist because he's a hypocrite – he sentences Claudio to death for fornicating with Juliet but turns around and tries to blackmail Isabella into sleeping with him.
We know what you're thinking – didn't we just say the Duke can be considered a protagonist? Yep, we did. Here's the thing, though. If Angelo is an antagonist because he propositions a wannabe nun, then the Duke is guilty of the same thing (unless Isabella has suddenly changed her mind about wanting to be a forever-virgin when the Duke says he's going to marry her).