Under the Canons of St. Basil the Great., those who killed in battle were required to abstain from communion for two years (the canonical recourse for murder). This should not be seen as a punishment or a penalty, but as an opportunity for "metanoia" (often translated as repentance, but better understood as a "change of heart" or "conversion"). Through prayer, fasting and abstention from the sacraments, the soldier is brought back to God, healed of the wounds to mind and soul, and reintegrated into the Body of Christ. It seems to me a more realistic and less cynical approach to a difficult moral quandary than saying "Killing is wrong, except if you meet the following criteria".