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.
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.
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.