So, I'm going to put in my two cents on this because i feel a few things are over looked.
Edelgard IS Rhea. Now what i mean is that El does the CF route she is effectively doing what Rhea did WAY BACK WHEN to Nemesis. So what i infer happened is that over time humans as their populations boomed in comparison to the dragon folk (forgive me for not using the in game species names but they arent super important) and had to spread out. As they spread out reverence/friendship for this undying species slowly became fear that this god like race might get tired of them since they so vastly out number them. This led to humans learning things in secret to siphon the dragons powers and use it agaisnt them. They also wanted to make it clear they meant business, so they targeted the broodmother first, Sothis.
This series of events led by Nemesis is what led Rhea (at the time as Seiros) to rally a group of people to fight agaisnt a potentially unfounded (again VERY little is actually said about this time its very possible some of the dragonkin were oppressing the humans) rebellion, they won Rhea shacked up with a noble person (this is canon in one the books in the abyss and vitally important to a point i will bring up later) cethleann took her nap etc. Over time to keep some control over who had these crazy toys that Nemesis' team had made she rewrote history, because of the weapons origins she couldnt either bring herself to destroy them or just simply couldnt find a way to destroy them, she knew she needed to stop them from being abused, thus the 10 elites and the crest system was implemented.
Now this is when things start going wrong and why being immortal causes problems because losing her mother creating a massive conspiracy, that also spawned a church that features YOU as the patron saint to keep people from causing further problems did something to rhea, she became absolutely obsessed with resurrecting her mother, even resorting to living sacrifices, and other dark magics such as creating homunculi. She also becomes uninterested in the petty affairs of men so long as her crest system remains in place they play by her rules but still prohibits free thinking (see loog rebellion). She also keeps fodlan on lockdown to prevent her false history from getting out.
Now lets to get to El. El is a bastard child to the emperor, and should have had no real place other than to be married off to someone lile ferdie or hubie, however her uncle had connections to people who survived rheas war and had slowly been sciencing is way to turn a living being into a monster that could outclass rhea. This results in children being kidnapped in all three regions and Els whole family being sent to the operating tables for 1 very important reason. They had blood relations to rhea because they have her crest. Her romance with that human who stood by her is what resulted in El, which in a weird twist makes Rhea the Marth to Els Lucina. During this time Els uncle staged a coup using his position within the TWSITD to manipulate/influence the other major nobles of the empire.
Sorry this is very wall of text but Ive noticed a lot of this info is ignored with all the Edelgard is x or y or Dimitri was right topics. El is a bomb whos fuse was lit by her uncle when she succesfully survived inhabiting 2 crests by the start of the game itself she has ~ 10 years to live and she knows this. She is also aware that her uncle can very easily remove her from power and take over embarr himself and use it to further twsitd plans and use them as cannon fodder. She is forced to play along with him or risk her people that she actually cares about, which drastically reduced her options. She didnt start this war it was started over a millennia ago by her xxxgrandma and a guy who feared her power, she just knew she needed to end it and fast and topple a system that had grown corrupted because rhea didnt stop the nobles from abusing power they really didnt control. What makes the CF so interesting is that Rhea in an attempt that at the time was a good idea but because she went crazy She actually PROVED nemesis correct, that her undying led to mass oppression albeit at their own power lusting hands.
Tl;dr Rhea lived too long to be the hero and died being the very thing she sought to prove wrong. She was put down by her own decendant attempting to fix the issue and learning that no one should hold power just because of arbitrarily gained genes boogied out of the crown asap. Was Els choice "the best" no but she had limited time and resources. She wanted to let rhea live (offers twice). Was Rhea evil? No she had good intentipns at first but became disillusioned and went coo-coo. Had rhea been willing to admit to alterong history and divulged the truth about crests and the "divine" weapons she could have joined El at that point and the 2 of them could have squished the twsitd and everyone loved happily ever after except bernies dad because we hung him at the gates of embarr for reasons
