Without wishing to seem rude, they clearly weren't paying very much attention to the "Thou shall not kill" bit of the bible!
Don't think that the bloodshed taboo is all that ancient or quaint; it was a significant concern for , say, 18th century surgeons.
It doesn't say "thou shalt not kill unlawfully" or "thou shalt not kill without social sanction". It says "thou shalt not kill". Period. Nowhere in the Bible is any condition or qualification placed on this commandment.
Reconciling this commandment with all the killing in the Bible that God has no problem with is not as easy as you want it to be.