All wars are neccessary. One side has an issue they can't live with, and the other side defends itself. The war of good Americans killing other good Americans brought home a specific truth. What shape would we be in today if that war had not been fought and won by the north? Would the south have found it's own path to not treating humans as property? No. I'm a southerner, but if I had lived in that time, I would have been in blue. The south had to be forced into compliance with basic morality. We can preach that it was "just the way it was back then," but never anywhere in the world was anything more unjust. It's on the same scale as the holocaust. The south would have struggled harshly without slaves, and the big plantations would have failed or not even existed, but the idea of keeping any person against his will HAD to be stopped.
The seizing of Africans for the markets was evil at best. If there is profit available, all men, white or black, will sell their souls to get it. That hasn't changed. What has changed is the legality of it. And we needed the war to put that part of criminality to rest.
That said, America is in worse condition that in those terrible times. If the slave traders hadn't brought them here, maybe we wouldn't have the racial conflicts that have divided our nation a second time.