Well i think its important to recognize that the commandments God gave to the Israelites before Christ, were to build His "nation of priests", the nation of Israel was to be the light of the world; however they fell because they would not heed the Lords commandments, they failed to completly eliminate the evil nations within the promised land, they failed to worship the Lord according to the commandments He gave to Moses, they mixed with the heathen and as a result, forfeited Gods protection of the land and of the hebrew nation... with Israel and Judah being summarily dispersed.
Christians are called such because they follow the teachings of the Christ, and as i recall, He commanded us to turn the other cheek. To love the Lord God with all our heart, mind and soul; and to love our neighbor as ourself. I find it hard to believe you love someone enough to kill them.
When the safety of our families are at stake, defending them goes without saying... but the only time killing someone to save someone makes sense is when our selfishness comes into the picture. Would you kill your children if they threatened your wife? would you kill your husband if he threatened your friend? these scenarios are no different than killing anyone for anyone. when you look at people from the eyes of Christ (which those who have accepted Christ can do because He lives in us and taught us how to see) then you will see that there are two kinds of people, those who have accepted Christ and are awake, and those who still asleep and do not know Christ. If i had to choose between the two, as horrible as the situation always is, i would rather a person in the knowledge of Christ be sacrificed than kill a lost person. if you would send someone to eternal seperation from God to satisfy your personal happiness, your desires... may the Lord have mercy on your soul, for you did not to your enemy.
i was in the military for 6 years, a great mistake of my own doing. but even in that mistake, the Lord was able to use to teach me about Himself. By the grace of the Lord i was not put into the position of being ordered to take a life... something i do not believe i would do even if ordered to do so.
So, i think to answer the OP's question, the best way to serve the Lord is to not put yourself in a position where you would be asked to take someones life.