They are most definitely antagonists and our allies are perfectly justified in their dislike of Garlemald however I still won't say they are just 'evil'. As mentioned antagonists are generally more fun as if they are well designed they tend to be more morally grey than just the 'chosen one' level of good like our warriors of light are and even makes more sense.

I doubt we'll get a choice as lets face it, no final fantasy ever would give you a choice like that but if we were I would certainly opt into the empire.