I actually believe that Flight 93 was shot down by the US airforce in order to stop it reaching it's target. The good ol' American passengers fighting back is a much more palatable story though (although if I am right and the plane was shot down, I can see the governments reasons. If they had managed to shoot down the 2 planes heading for the trade center, forcing them to crash land in a field outside of New York City, world history would be very different).

This has little to do with the 'is violence justified' discussion, but it does relate to the 'greater good' sub-discussion.