In many and indeed most cultures, the killing or assassination of enemies is sanctioned and sometimes even encouraged in their warrior traditions.
For example, the Bushido holds that the warrior is as a cherry blossom - brief and beautiful and doomed. Comparable to the beliefs of the Amerindians whose mantra of "Today is a good day to die" propelled the warrior into a state of selfless focus and allowed him to kill guiltlessly without fear of death.
Consider the desert assassins from Arabia and Africa - their technique was to become the wind and the sand, assuming an elemental aspect similar to the Shaolin monks whose reverence for life is suspended while they were a force of nature.
Of course, in a lot of cases, such as the Incas and Aztecs, killing became ordained. Made holy by sacrifice and sacred duty. As with the chivalric knights Templar, a vigil or prayer and fasting is observed before the killing, making it a de facto church execution thus guaranteeing absolution from sin.
Mr Lurksalot has his own philosophy: