Okay, but to be clear, it is not a question of pacifism.* It is a question of whether there could ever arise a case where we think about killing someone, but first we must check and see whether they are a Christian, and then if they are not a Christian we can go ahead and 'off them. Your OP seems to assume that there do legitimately arise such cases: that there are cases where we would be permitted to kill someone except for the fact that they are Christian.
*
@Ace777 quoted me out of context, which is dishonest and unfortunate.
My thinking when raising the question was the Lord's High Priestly prayer; and following that the context of how Christian attachments to the temporal powers has resulted in a Church broken by national borders; such that Christians are willing to kill one another if ordered to do so by king and country.
Perhaps I should have offered a more direct question in the OP, however I left it intentionally open and vague for the purpose of seeing what kinds of responses would be given in a more open-ended question and discussion.
To argue my own position: I believe that the Christian's allegiance is always foremost to Christ; and such allegiance to Him as Lord means the unity of the Church in familial affection; and it also means to regard all my neighbors as the objects of my Christian affection. Whether that neighbor is on friendly terms with me, or even if that neighbor is hostile toward me for the Lord commands, "Love your enemy" saying "what does it profit you to only love those who already love you? Do not even the pagans do that?".
So that because of the Christian's allegiance to Christ as his or her Lord, it follows that I am to regard all other people as neighbor toward whom I am compelled to love; this is especially true of those who are also in the household of faith.
So in this I am opposed to violent action in general, and believe in choosing non-violence and non-violent solutions as a matter of moral imperative, and principle. I confess that there can come times when, tragically, violence is necessitated by extraordinary circumstance; in which case it may be justified in order to prohibit greater evil. If I know that someone intends to kill many innocents, and if my only option is to violently stop them, then I believe such is justified. I consider this an extraordinary, rather than ordinary, act however.
As it pertains to larger social conflicts, such as war; I believe that the Christian ought to abstain from swearing a soldier's oath; as our holy fathers did in ancient times and for which they were often subjugated to the cruel tortures of tyrants--through which they gained their martyr's crown.
As St. Cyprian comments, "
The whole world is soaked with mutual blood; and murder, which in the case of an individual is admitted to be a crime, is called a virtue when it is committed wholesale. Impunity is claimed for the wicked deeds, not on the plea that they are guiltless, but because the cruelty is perpetrated on a grand scale." - Epistle to Donatus
Again, the question may be raised in regard to greater evil and social responsibility toward it; hence we can speak of just war doctrine; or perhaps more rightly, permissible war. The great example of our collective memory would the second world war. To which I add one comment and one question:
1) That the temporal powers are right to act to curb evil, as St. Paul writes in Romans ch. 13; and thus should the temporal powers wield the sword in war for that purpose--it can be regarded a justified act. But only insofar as it is, indeed, justified for the purpose of curbing evil.
2) To what degree the Christian may participate; and even more importantly--the Church's witness in the midst of such conflict. For should the Church, who being the catholic Body of Jesus Christ comprised of all nations and subject to no temporal power for she is subject to only Christ her Royal Head and Lord, ought to divide herself along the boundaries of the map; such that the Church when she is found in one place may side with the powers there; and when found in another, may side with the powers there; and thus her children, the Faithful of Jesus Christ and children of God, called to take up arms on behalf of the temporal powers and inflict punishments against one another.
Should we not consider it, in this last question, the utmost absurdity?
-CryptoLutheran