King Miraz is one villainous dude. Prince Caspian claims that Narnia under his uncle is an "unhappy country" since the "taxes were high and the laws were stern and Miraz was a cruel man" (5.2). He also used political trickery to remove his political opponents so he could stand unopposed in power.
Okay, fine. But does all this make him a villain or just a politician?
Well, let's not forget that Miraz kills Caspian's parents so he could acquire the throne as the regent. Then he tries to have Caspian murdered so Miraz's son can claim the throne that is Caspian's by right. So, yeah, we're going with villain.
But Miraz is more than just a villain; he's also an antagonist. True, antagonists are usually characterized as world-conquering bad guys with minions, who live in castles complete with torture chambers (which Miraz totally has). But that's not what makes him Prince Caspian's antagonist. Instead, it's the fact that Miraz stands in opposition to Prince Caspian by being everything Caspian and his friends are not.
While Caspian loves the stories of Aslan and Old Narnia, Miraz hates them. "That's all nonsense, for babies" were his exact words (4.12). Peter courageously challenges Miraz to a duel because it's the only chance the Old Narnian army has. On the flip side, Miraz accepts the challenge, but not out of courage. Oh, he thinks it's courage since "[e]xcuses for not fighting" are cowardly, but when "all good reasons of captaincy and martial policy" say you shouldn't fight, then you darn well shouldn't. Miraz's brand of courage looks more like foolish machismo to us.
To sum Miraz up nice and neat: villain, antagonist, and all-around bad dude.