Thou Shalt Do No Murder
Murder is a subset of killing.
Hardly the scriptural scholar I'd like to be, but isn't the translation more appropriately "do not murder"? Murder reflects perverse motivations -- unearned gain, lust, revenge. Murder is a subset of killing, not its equivalent. Killing may be always awful, yet justifiable, in defense of self, others, groups as in the nation.
There is a time to yield, avoid conflict, turn the other cheek, put away the sword, die under a Chinese tank, or on a cross. Some would say that those who truly take on the full mantle of apostleship of Christ on earth might even eschew normally-excusable self-defense. However, if employing lethal force seems the only recourse for such a horrible prospect as defending the innocent from a threatening sociopath, I expect that the Judges on High can differentiate the sin of murder from the horror of a self-defense killing, at least as well as any good judges on earth.
The ugly but necessary right to lethal self-defense, extrapolated to the group, means civil police and national armies. This sometimes means scenarios on a large scale -- war -- that reflect the same ugly but necessary value. Although war, in practice, is more the realm of the guy with the horns and hooves than anything of the realm of harps and hymns, those who fight for the good causes on the front lines are as justified as those who defend a single innocent from a maniac.
This sensibleness regarding "do not murder," as with all the commandments, derives from there really being just one law -- as Jesus well emphasized, the love of God as the Parent, and the inevitable consequence, filial love of one's fellow. Murder from sinful motive is loveless, the killer's ego vaunted, the victim devalued. Conversely, by the measure of the golden rule, that which is done in a necessary service of defense is done in love. This can include loving as best one can that poor confused soul dispatched of necessity to higher judges. And, since so many have fallen in defense of others, laid down their lives for others, recall that Jesus said, there is no higher love.