Is he a utilitarian? Because if he comes to libertarianism from the starting point of rational ethics, it's not hard. That principle requires one to accept that the ends do not justify the means. The ends must be justified in and of themselves, or it is still unjust.
Try this analogy: what if I shot a bullet through an innocent man's
head, and it kept going and struck and stopped a man in the act of
burglary? Does that make the act of my volition any less a murder?
That's absurd.
It's simple logic: If A does not equal B, A CANNOT equal B. A cannot become B unless A's nature is altered fundamentally so that it becomes B.Injustice is not made just simply because a side affect of it can be considered just. The action itself must be just.
And even if I had been aiming for the burglar and hit the innocent man, I would still be responsible for the unjust death of the innocent man.
Taking it on from a utilitarian perspective, the fallacy of the broken window applies here. The war killed hundreds of thousands, destroyed vast amounts of wealth, and as an unintended consequence, slavery was ended. But it was completely possible (likely, in fact) for slavery to have ended without those losses. From this perspective, too, Lincoln's war fails to find justification.
Pro Christo et Libertate integre!