I had always heard that Lee had a crisis of conscience at the beginning of the war because he despised slavery but couldn't bring himself to fight against his homeland. Then recently a friend told me that Lee had said things about the necessity of slavery to condition the blacks, or some such thing. Can anyone shed some light on this for me?
This comment covers the entirety of your point, and what your friend told you about it.
Essentially the point is this:
Lee didn't want to free his slaves, and by some accounts was very harsh with them, even though the person he inherited them from stipulated in their will that they ought be freed.
He still did fight against his family, even if not his homeland. He still fought against the United States by committing treason, which was his homeland regardless, and still makes him in the wrong.
He believed, according to a letter he wrote to his wife in 1856, that slavery was a moral evil but that slaves should be freed by "Providence", and that abolitionists were acting against God and acting in a way inconsistent with the "master plan" of Providence, as he put it. He also thought they were better off slaves than anywhere else, and that they'd become better by being "instructed" as slaves rather than otherwise.