You didn't have to die. A gentleman opens doors for ladies and lets them go first.
and then you died.....unable to ever leave the Purifier control room.
Game over man!
Seriously though, that is equally stupid when you have Charon and Fawkes in your party.
You can, apparently she also dies in Broken Steel (I never did it on a BS playthrough) if you send her in instead of you or your totally impervious to radiation companion in, but Vanilla Fallout 3 pretty much acts like you die either way.
Unsatisfying either way, thankfully Broken Steel allowed me to retire my LWs as I wanted.