When is slaughter acceptable?

Viewing some news mini special of the events of 9/11 and who was really in charge out of G.W. or Chaney it was revealed that any jet that could not be contacted the military had permission to shoot it down. Almost, as if the passengers on those jets were already dead, making them dead quicker was OK. To me it was like saying the slaughter of those hundreds of people on any jet unable to make communication was acceptable.

That made me think of how people have viewed certain incidences like the V-tech shooting, Columbine, the San Ysidro McDonalds massacre, etc seem to have a palpable effect on people. They are horrified by the gratuitous loss of life. I think how come, because there was no presumed gain from it? Is it because they are fellow US citizens and thus seen more valuable? When Ruanda happened I knew people were shocked but many didn’t seem outraged, nor did they about the slaughter in Darfur. Not on a great scale anyhow. It seem like more people were concerned over Darfur than the fire and carpet bombing of Dresden and Tokyo where tens of thousands were slaughtered overnight, the majority of which were civilians asleep in their beds. Are people of the US more jaded to slaughter or spilling blood of the innocent if there is apparent gain behind it? I remember reading in Time that when they struck down Abu Musab al-Zarqawi they killed three women and a child with him that had nothing to do with the insurgency, they were just at the wrong place at the wrong time. No one seemed to bat an eyelash. It was as if it was acceptable in order that a-Zarqawi not escape. Yet when a single child dies and is presumed murdered like Caylee Anthony many get bent out of shape. Are people more jaded to slaughter when they believe the gains are worth it, is that it is the means to an end?