How do you invade another nation unprovoked, refer to your opposition as "creatures" to dehumanize them, and vaguely hint at genocide and feel like "Yeah, I'm the good guy"
Well I mean Rhea totally is a creature!! You see with your own eyes right away when you turn on her, she turns into a crazy beast! Then it turns out she's been controlling all of humanity in fodlan for a thousand years, by violently murdering anyone who even thinks about standing up against the church, and you see some of that in part 1. And then there's all those crest experiments they seem to be doing on Lysithea, and Edelgard's family, and who else... crazy stuff.
And as for why it's ok to invade another nation unprovoked, it's a revolution, she wants to get rid of the unfair nobility/commoner divide, and stamp out all supporters of the corrupt church; replace the society with a new society, a meritocracy where science is held above faith. Hell yes! She's the hero brave enough to do it. ---
People who don't finish their sentences I'm a Castlevania superfan! Ask me anything!