The morality of war
The following quotation is taken from Fighting for the Confederacy, a personal memoir of the War for Southern Independence by General Edward Porter Alexander, who was a 26-year-old captain of engineers in 1861 (the time of this anecdote).
The young Alexander's prim correctness is rather amusing, and perhaps he was right to disapprove of Col. Cash, who may not have been an admirable fellow. But in one respect the choleric colonel was way ahead of his time: in recognizing that war is the greatest of all crimes, mass murder beyond the dreams of any serial killer, and politicians are the criminals responsible for it.
If anyone deserves killing in war, it's the politicians who authorized it and who could stop it at any time. The usual convention that politicians are to be treated with respect and not made military targets derives, of course, from the politicians themselves, who agree to make each other immune from the bloodbath they have created. It's not clear to me why non-politicians should respect this convention.
Justification for war
Can there be any justification for making war? Yes, surely. If an army invades your country and begins to kill people indiscriminately, without provocation, then I think you have to regard that army as an organization of murderers, and return fire in self-defence.
But this is a rather extreme and easy case. In general the case for war should be examined very sceptically, given the vast amount of death, suffering, and damage it causes.
Furthermore, the use of conscription in modern wars is very hard for a libertarian to accept. We might reasonably say to the politicians: if you want a war, fight it yourselves! Or at least pay volunteers to do it for you.