I think one of the things I find very compelling in Edelgard's route is how many games, but in particular Fire Emblem games with their focus on "permadeath" making it clear you're not simply "knocking down" your enemies but are actually killing them, is how this route acknowledges the horrors of war and the responsibility of your actions.
In most game plots, it's all laid out so that, yes, you may be a mass murderer, but "the bad guys left you no choice", which serves as a "feel good" justification for the player to engage in mass slaughter of enemy soldiers.
Here the game doesn't try to hide that you're doing this for selfish reasons (even if the reasoning behind it is motivated by selflessness, i.e. "making the world a better place"). In the end you're making it for your very own views of how the world should be (assuming you actually agree with the protagonist and aren't just playing the route for "gotta catch'em all" reasons). (*)
There's no mental gymnastics about how the player is "in the right", 100% a good guy for killing numerous other human beings. War is ugly and the game doesn't try to pretend you have the right to have fun killing other soldiers because "you have nothing to feel wrong about, no reason to blame yourself".
I guess in the end I'm just getting old and tired of games telling you "it's cool if you kill people because you're the good guys, and good guys have all the rights in the world".
Anyway, if that post ever makes sense.
Edit: (*) This also mirrors how the player plays the game for his/her selfish entertainment reasons.