It's a tricky situation, philosophically. This may have helped the UK abolish slavery sooner (a good outcome), but its morally and legally dubious.

This was suggested many times during the lead-up to the Civil War, but (I think rightly) Lincoln as well as the more hard-core abolitionists viewed this strategy as contradicting their position that Slavery was a crime against Natural Law.

Lincoln's strategy required the forced labor (and for some, subsequent deaths) of tens of thousands of people. The irony of conscription wasn't lost on Americans at the time either, and the military drafts were extremely unpopular on both sides.

Lincoln's strategy itself was "required* because slave-owners and slave-owning states would not give up their slaves without a fight. Blame them, not Lincoln.

Lincoln himself was well aware of the costs that would be imposed by war, but chose this course of action due to the absurdly perverse nature of slavery.

http://www.bartleby.com/124/pres32.html

I don't doubt that Lincoln personally resolved his internal ethical dilemmas, but I do doubt that this would be any consolation to me if I were forced to fight in an army.

As satisfying as it might seem to just shoot the slaveholding bastards instead, that ends up being even worse in humanitarian terms.

You are aware, that the first slave owner in Virginia, who went to court over his slave, was a black man, right?

How does that change anything I wrote? White slaveholders held white slaves in ancient Rome but it would have been justified to kill them all as well, too.