The Inhuman Royal Family are oppressive tyrants who consolidated their power through an unfair caste system. Not only that, but they continuously isolated and mocked Maximus simply because he was not transformed by the terrigen mist.
Maximus was the real hero of the show (or he was until the last couple episodes). He wanted to abolish their oppressive system and go to Earth to save their civilization from a lack of resources. Not only that, but going to Earth would have allowed Attilan to establish a sanctuary for all the new Inhumans during the Inhuman Outbreak.
I think the writers eventually realized that they had accidentally made Maximus’ motivations more noble than the protagonists, so they had to “character assassinate” him so that the audience roots for the “heroes” instead. They had Maximus do a complete 180 and go against his entire character to make him want to destroy Attilan for absolutely no reason. His entire character was that he was a populist liberator for his people.
Furthermore, Black Bolt and his fellow dictators are continually selfish and aggressive to anyone who is not with them throughout the entire show.
One might argue that since the Royal Family ended up settling on Earth at the end of the show that they were redeemed, but this is false. The only reasons they resettled in Hawaii is because A) Attilan was already too far gone and they had no other choice and B) The movement to move to Earth had already been started by Maximus, so there was no undoing that ideology among the people. The Royal Family migrated to Hawaii because if they didn’t, they would have surely been overthrown. They came to Earth to save their own skin, not because it was the right thing to do.
These guys are definitely the villains.
Edit: Lol except for that stuff where Maximus kept hitting on Medusa. That was incredibly creepy.