Countries have laws against as it is 99.9% of people's opinions that it's wrong, and that it's necessary to keep peace. These people don't consider it wrong, therefore it cannot be fact - it can't be fact if there isn't 100% of people who consider it wrong.
I'm sure cannibal's also considered it fair enough - they had a reason, yes, but so did the terrorists, however bad the reason was. It's our opinion now that that is very wrong, but it was their opinion that they were right to kill (and eat) other humans.
War doesn't decide who's right - just who's left.