In a conflict, both sides of a war believe they are right regardless of who is actually in the right/wrong, so modern perspectives are skewed by who won the war. As they say, history is written by the winners. Knowing that I am somewhat biased because I am the result of what happened, I'll try to answer this as unbiased as I can.

Both sides of the war considered themselves to be correct. Similarly, both sides looked for confirmation bias in the past. GW was probably a slave owner, but he was also a fan of the ideas of freedom and the likes. The two sides were taking specific ideas that he presented and utilizing them to look better for their argument. It's like how modern day Redditors like subs that they like because they can have confirmation on ideas that they themselves slightly question. People go to /r/femenism because there are like-minded people there, but some people go there to cause commotion and disagree with the ideas presented.. The two sides of the Civil War looked to GW as being reason for their fight, because by taking specific isolated examples, they would be able to create the argument that they wanted to fight.

I probably sound crazy because of how late it is. I'll read over this again in the morning to make it more understandable.