Wow, I am new to this conversation, and I haven't read everything because it is too much info, but I got some info of people saying that Edelgard is on the wrong side, that even though her ideals are good her methods are wrong.
And in the end you guys might be right. She did a lot of things that are morally wrong, making her definitely a villain character. However, we need to consider that siding with her makes total sense, especially if you are going for the S support with her. We know that Edelgard won't change her mind even with you side with her, so the only thing left for Byleth(avatar) to do is to join forces with her and walk down this dark path with her.
We see Edelgard dying by the lance of Dimitri in the BL route and Byleth didn't even care. We see Byleth killing Edelgard in the Church and in the GD route with some hesitation, but in the end he did what must be done. However in the Crimson Flower route, we can definitely see that Byleth is too fond of Edelgard, and so is she with him. The chemistry between the two is so strong(much stronger than with Dimitri and Claude in their routes) that siding with her makes total sense. That happens because, If you can't change the mind of the one you love, so join her in the path to hell. At least that is what I think, and that is why I find this game so good and tragic at the same time.
I played the Crimson Flower path first, and I grown too fond of Edelgard. To me it does not matter if she is good or evil. But not having her as my lord and having to face and kill her in the other paths makes the other paths much harder for me to play.