...

you: hey how are you doing?

me: the reason why the fea ending hits so hard is because they give you the choice. If you removed the "everybody lives (including grima whoops)" ending you would have a much lesser impact. The game offers you a choice, offers you the chance to keep yoru character, that character you grew attached to for hours and hours of gameplay, alive, and if you want to do the right thing you have to be the one to choose to sacrifice yourself. It's not a scripted event. It's not a "if you don't do it you get a bad ending." Nothing is forcing you. You sacrificing yourself is up to you, and you alone, you, and your moral compass alone. And that's why it hits so fucking hard.

you: there's also the fact that this is a simple, straightforward choice. The game doesn't give you five escape routes the way dragon age does. do you sacrifice yourself? yes, or no. the world is ending. there is no time to argue. there is no alternative, no secret third route, no way to end this with anything but blood. will you sacrifice yourself? yes, or no?

me: YOU GET IT, and though the previous choices in the game (lucina's judgement + your promise to chrom) don't actually matter much, they're here to get you used ot the idea of choice, of agency. Chrom begs you to live. Lucina asks you to die. Two characters who give you a choice. Two characters who are your choices at the very end.

you: tea or coffee by the way?

me: tea.

you: wrong it's coffee