She called Rhea a monster because she let Fodlan go to hell thanks to the Crest based society, the cool almost complete power she has and not doing absolutely anything to solve the issues that cropped up because of crests (like, you know, the experiments she went thru)
Rhea sure did all of what I described already and also Edelgard lets Sethet and Flayn go without issue so it's just against Rhea (and the people who follow her).
Also weird that she calls Nemesis just a bandit in Verdant Wind but does call him King Nemesis in Crimson Flower when she's alone. Probably nothing.
I mean thanks to everything that has happened to her, the moment Byleth opposes her she's basically never going to want Byleth to save her.
Also three houses is good because it sure loves to tell you to live with your choices.