Imagine being legally allowed to kill someone.
Oh wait, you don't have to imagine, it's called war, we train people to do that.
Discussion
Yes, it's weird how we compartmentalise human actions depending on who's doing it and to whom. The reptilian part of our brains is the part that is nurtured and honed from an early age by schooling and propaganda.
Natural law does not in fact allow us to kill anyone. Natural law is the only real law. Everything else is just a concoction of rules and regulations designed to keep humanity subservient to those to whom voters defer responsibility.
It's OK to kill other people as long as you do it in the name of King Charles the Turd.