Question., should Christians be in the armed services?
IMO there is not a definite yes or no to that question. It would depend upon circumstances. But to the question if violence can be justified under some circumstances, then I think the Bible is very clear on the positive.
As the law of God (yes, that old despised law that Jesus said He did not come to destroy, even the five books of Moses) defines what is sin, and where there is no sin there is no transgression. This gives:
1. To use violence, to use weapons, even to the point of lawfully killing the guilty, is not a sin at all. In fact it is commanded by God in some cases (Gen.6:9).
2. The primary source of using violence should be with those that have been authorized to do so, as implied both in the law and by Paul.
3. The law does not allow for self-revenge, but for self-defense (Exo.22:2).
The huguenots in France had armed guards at their meetings (to defend themselves from papist raiders, who would have killed them if they could). An example of self-defense.
I will add that I believe that we are only bound to be subject and obey
the powers that be in as much as these in their turn fulfill their God-given duties, namely to be ministers for us to good, a terror for evil works and a praiser of good works.
Also, we are only to render honour and pay taxes for aims that are lawfu and due. Not to support things like abortions and other abominations. Yes, they force us to pay taxes for those ends anyway in most countries, which is unlawful and those responsible for it will give account.
Rom.13
[1] Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained of God.
[2] Whosoever therefore resisteth the power, resisteth the ordinance of God: and they that resist shall receive to themselves damnation.
[3] For rulers are not a terror to good works, but to the evil. Wilt thou then not be afraid of the power? do that which is good, and thou shalt have praise of the same:
[4] For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil.
[5] Wherefore ye must needs be subject, not only for wrath, but also for conscience sake.
[6] For for this cause pay ye tribute also: for they are God's ministers, attending continually upon this very thing.
[7] Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour.
Matt.22
[20] And he saith unto them, Whose is this image and superscription?
[21] They say unto him, Caesar's. Then saith he unto them, Render therefore unto Caesar the things which are Caesar's; and unto God the things that are God's.