So I've been thinking about the ending. Again.
[spoiler]It seems to me that the whole "It's either you or Sarah" thing was thrown in at the last minute. If you pay attention throughout the game, it looks like they're building up to Sarah being the one to get blown up. I mean, think about some of the things she and her friends say (from memory but)...
"She died well. In the end, that's all that matters."
"We're all willing to give up our lives for the people of this wasteland, if it comes to that."
"We in the Brotherhood believe that how one dies is just as important as how one lives."
So yeah, Sarah Lyons is totally fatalistic and at that point should be jumping happily into the reactor saying, "Hells yeah! Gimme that code! This one's for you, Morf! AH-CHOOOOO!!!!!"
EDIT: Well huh. While I was Googling around, I actually found a confirmation of this, straight from the horse's mouth.
(quote from rpgwatch.com/forums/showthread.php?t=6130) (quotes and spoiler tags don't like each other, sorry) (and neither do spoiler tags and proper URLS!)
We'd only ever planned for you sending Sarah Lyons into the purifier, because we knew, from a story standpoint, that she'd definitely be in there with you.
(/quote)
And I guess at the last minute they decided, "Hey, just to add some drama, let's completely change Sarah's personality so that she suddenly places her own life above yours (despite you being both a 19-year-old innocent Vault kid and single-handedly accomplishing more in the fight against the Super Mutants in a few months than the Brotherhood has ever) and make it so that the narrator chews you out at the end if you decide to preserve your own life like you've being doing throughout the entire game."
This is why moral dilemmas and interactive mediums don't work, people. It seems like the perfect use for the format, but inevitably you're going to corner people into a false choice at best and a stupid false choice at worst.[/spoiler]