Miraz is a villain of the moustache-twirling variety. He uses dirty politics to wipe out his enemies, he is cruel to the Narnians, he tries to have his young nephew murdered, and he owns a torture chamber. Yeah, not a nice guy.
But that's not the reason he's the antagonist. He's the antagonist because he's the person in conflict with our protagonist, Caspian, and their conflict is the main driving force of the story. His villainous ways are just icing on the evil cake.