12th June 2010, 10:17 PM
You can't kill anyone who will impede the story, they have a "protected" status. You can knock them over when their HP hits zero but they won't die. I don't honestly find myself knocking NPC's off of cliffs too often. Unless you're leaving a trail of blood in your wake as you leave Megaton or Cheydinhal, it's not a huge concern.
The Earthworker Race has ended. Everybody wins.