The avatar delivered the final blow, best ending IMO.
I wish there was extra text when Chrom has you promise him whether or not you'll deliver the final blow if you married him. He made a large deal about wanting Lucina to grow up with her whole family, and yet he doesn't really care that the mother sacrifices herself. I wish he said some of that, as it would have made that moment at the end much more dramatic since he already had negative feelings about the avatar being in favor for the sacrifice.