Throughout the war, for all except a very few far-seeing southerners, notably including R.E. Lee, the protection of slavery was a non-negotiable war aim. Just weeks before Richmond fell the Confederate Congress was still balking at emancipation even of Confederate black soldiers .
As far as "might makes right" goes, how about this: Those who choose to pick up the sword to settle political isues may regret it. Christ said that those who live by the sword will die by the sword. Might does not always make right, but neither is the winner in war always in the wrong.
On what basis other than "might makes right" can slavery possibly be justified?
FYI, one of the arguments against using slaves as Confederate soldiers that appeared in the 1865 newspapers was that if that were done, there would be no one left to plant and harvest the crops and that the resulting food shortage would be far more damaging than Grant. Indeed, there was a shortage of food already among the Confederate troops, and Grant's soldiers gave them food after the surrender.
This is exactly my point. Eventually, in any discussion about the Civil War, eventually all Unionist arguments essentially descent into "slavery is an absolute wrong, thus the war justified."
But that seems to be rationalizing, because everyone agrees, including yourself, that slavery wasn't the purpose of the war in the first place--it was purely to preserve the Union, which is a direct slap in the face of the concept of self-government. Lincoln was willing to kill or cripple millions of people, waste millions of dollars, and destroy the economy of half the nation (for at least decades, maybe a century thereafter) all for the supposedly grand purpose of "preserving the Union." What purpose did it accomplish?
I'm not defending all of the South's practices, but if it decides that the United States--States United--isn't working out for it, then it ought to be free to leave. Likewise, if California or New York or whatever decides that it just doesn't like the conservative course that the rest of the country is on and it wants to leave, I bid them farewell.