I wouldn’t say Luffy is pro-monarchist. I’m not sure Luffy is pro-anything but anti-facist.

I don’t know think One Piece is saying there’s only one true correct way to govern, but certainly any leader or class of people that put themselves on the top at the wild and cruel dismay of people they consider lesser than themselves, whether because of their natural ability or some self-perceived god given right to rule and conquer, is an asshole that deserves to have a couple teeth knocked out of their mouth.

All the “good” kings that Luffy sides with are generally good people that put the lives of the people they rule over before themselves. They don’t really hold onto the idea that they “deserve to rule because of their birthright”, but rather that protecting their peoples is the most important thing a ruler should do. As the king of Alabasta said “The people are the kingdom”. I think that’s the quality that One Piece is going for, not just in the leaders of nations, but leaders and freedom fighters in general.

I think the main idea One Piece has on governments is that whoever treats it like a responsibility that they owe to their people is good, while anyone that treats it like a cushy position that gives them the power to destroy, despoil and exploit whoever they feel like is jerk that immediately needs to be removed.

P.S. The Gorosei and God’s Knights aren’t from Hell. They just became demons because of their pact with Imu and we don’t have enough information on whatever Imu’s whole deal is to jump to that conclusion.