Slaveholders treated human beings like property - by doing that they lost any claim to any high ground.  Period!  Just because something is "law" does not make it morally defensible.  Check the Nuremberg trial logs.  War is war - civilians have always been casualties of war - just ask the British and Germans under the bombs - that's the way wars are fought.  It's an ugly fact but the enemy includes enemy civilians - look at what we did to Japan with nuclear bombs.  There is very little morality in war.