You really can't judge a 19th century culture with 21st century values. It was a different time. The entire world was just waking up to the horrors of slavery, and the fallacy of racism.
It's been reported that Robert E. Lee had been asked by Lincoln to fight for the North, but Lee refused, not because he supported slavery, but that he couldn't bring himself to fire upon his fellow Virginians. I suspect many who fought for the South were not pro-slavery, but they were pro-Georgia, pro-Tennessee, pro-Arkansas, etc. State's rights and state identity were much stronger then.
I further think that had not Lincoln pressed the issue, slavery would have died a natural death in the South, as it had in England, over the next few years, and the Union could have reunited without having to bow to the supremacy of Washington, DC. That's just an opinion. Who knows what would have happened. What I do know is that the Civil War was a disaster not only in lives lost, but with the resulting concentration of power at the federal level. Of course it was wonderful to end slavery, but I'm still not convinced that slavery was the real impetus for the war.