Bishop Bryan Ouellette introduces the topic of "institutional sin." It is the sin that results from necessity by the lack of virtue existent within the fallen world. The example of killing in war and self-defense is utilized as the initial point through which this difficult subject is mastered and thus understood.
Correction from Bishop Bryan: I had the Himalayan mountains on my mind, as I had just finished reading a story about some Buddhist monks. In my absentmindedness, I accidentally said I was coming to you live from the Himalayan mountains when I should have said Appalachian. My apologies for any confusion, but no, I have not yet set out to the life of a Buddhist hermit. ;-)