And the comment on the fair treatment of slaves is meant to mean that Lee might believe in that but within the context of believing that slavery isn't inherently unfair - the kind of guy who probably also treats his dogs well.
Minor nitpick: Most?
Theoretically.
Ok so we have theoretically, that's good that means we're getting somewhere. Ok, so Lee witnesses something that changes his attitudes twoards black people. It's going to have to be something pretty horrific to change the mind of a white southerner in the 1860s. Something along the lines of having a black man litterally ripped apart in front of him. Something where he is horrified, but everyone else shrugs it off, making him even more horrified. Who was the worst slaveholder in Virginia? That might be a good place to start.