I think, up until the point where >!Rhea orders you to execute Edelgard, I am probably on Rhea's side. Up until this point, my thoughts are, Edelgard is dangerous and needs to be captured and questioned, she's made some serious mistakes and moral transgressions. But once I'm given the choice between murdering one of my students or being branded a traitor, I'll side with the student every time, regardless of whether I thought she was in the right beforehand. There's no middle ground with Rhea, you are either with her or against her, and that moment is where it most clearly comes into focus.
In other routes you find out that Edelgard is not so different from Rhea in that respect, she becomes ruthless and if you're not with her then you're an enemy. But in siding with Edelgard, you soften her world view somewhat and help her to become a more judicious ruler.!<