Phrase to remember for OASC, it worked wonders for me when I was asked this question:
"It's a necessary evil."
No point waffling on about chains of command, rules and regulations or morality if you've got 3 seconds to decide 'whether to attack'. In the end, attacking and killing other people is not a mission you should enjoy, but it is a mission that needs to be done.
Thats about it.