I think the Smoke Monster let them live for a reason. I don't know what that reason might be, but as we all know there is a grander scheme, a big picture, a game being played that we don't yet understand. Still not sure why Smokie killed Eko (I wasn't paying close enough attention) but the pilot might have had information that the Smokester didn't want spread? In any event I think if Smoky or it's contoller (The Island, Jacob, Ben?) wanted th mercenaries to be killed, they would have been killed.