Aren't you projecting your 2017 values on those who were living in 1861?

Slavery was legal in 1861.

Genocide and invasion of other countries was not legal, per international law in 1939.  Your analogy is false.

Most Southern soldiers did not own slaves.  The premier Southern generals, Lee & Jackson, thought that slavery would die a natural death.  Indeed, their wills freed their slaves.

Lincoln only turned to slavery as an issue in the Emancipation Proclamation because he wished to keep Britain & France out of the War.  Lincoln freed the slaves in areas of the country where the Union had no control, but he permitted slavery in the Border States and those areas of the South which the Union controlled.

Lincoln and his cronies could have saved immense amounts of blood & treasure simply by purchasing slaves at auction.  But his goal was domination not emancipation per se.

Indeed, Lincoln wished to deport blacks after the War because he believed that blacks & whites could not coexist.