You said,
"We are going round and round here. Lee joined the Confederacy. That made him a suporter of slavery, even if he never said anything explicitly against it. Lee regarded Black people as inferior to white people - therfe are many sources substantiating this. His view of slavery as "a necessary evil," is often quoted."
If joining the Confederacy made someone a supporter of the institution of slavery, then almost every Southerner by your statement would fit the bill. You have applied guilt by association.
First as to Lee being a supporter of slavery:
"Considering the relation of master and slave, controlled by humane laws and influenced by Christianity and an enlightened public sentiment, as
the best that can exist between the white and black races while intermingled as at present in this country, I would deprecate any sudden disturbance of that relation unless it be necessary to avert a greater calamity to both. I should therefore prefer to rely upon our white population to preserve the ratio between our forces and those of the enemy, which experience has shown to be safe. But in view of the preparations of our enemies, it is our duty to provide for continued war and not for a battle or a campaign, and I fear that we cannot accomplish this without overtaxing the capacity of our white population." [Robert E. Lee to Andrew Hunter, 11 Jan 1865]
Full letter here:
http://www.civilwar.org/education/history/primarysources/robert-e-lee-to-andrew.html
As I read this letter I see a supporter of slavery who has reluctantly come to the conclusion that some slaves will have to be freed to keep the rest in chains. However, he does recognize it is possible this will lead to the collapse of the entire institution: "If it end in subverting slavery it will be accomplished by ourselves, and we can devise the means of alleviating the evil consequences to both races. I think, therefore, we must decide whether slavery shall be extinguished by our enemies and the slaves be used against us, or use them ourselves
at the risk of the effects which must be produced upon our social institutions."
His plan, though, is to
only free those who would fight for the confederacy and later their families: "There have been formidable armies composed of men having no interest in the cause for which they fought beyond their pay or the hope of plunder. But it is certain that the surest foundation upon which the fidelity of an army can rest, especially in a service which imposes peculiar hardships and privations, is the personal interest of the soldier in the issue of the contest. Such an interest we can give our negroes by giving immediate freedom to all who enlist, and freedom at the end of the war to the families of those who discharge their duties faithfully (whether they survive or not), together with the privilege of residing at the South."
Coming at this late date, January of 1865, it is obvious he has exhausted every other means of filling the ranks and this is his last option. "I should therefore prefer to rely upon our white population to preserve the ratio between our forces and those of the enemy, which experience has shown to be safe."
Secondly, as to the confederacy. Its own vice president said that its cornerstone was slavery and ****, which Lee in this letter is supporting.
Third, Lee voluntarily accepted an offered commission. He wasn't conscripted. Someone who knowingly joins an entity voluntarily then supports that entity's goals. Every southerner who joined the confederacy without being conscripted, then, made a conscious decision to support that entity's goals. The goal was independence
in order to preserve the institution of slavery.
When Lee was pacing back and forth in his house before sitting down to write his resignation from the US Army, I am quite sure part of his angst was not that he would have to fight for slavery. The available evidence indicates he wouldn't have a problem with that. His angst no doubt was over leaving the US Army that he had loved being a part of, possibly fighting against friends, and committing treason against the United States weighed against fighting against the south, possibly against his family in Virginia, and for a Republican administration. It is simply not reasonable to suggest Lee had no idea why the confederate states had seceded. Even in the oft-quoted letter to his wife he talks about the conflict over slavery, and there is no doubt that he is in favor of the proslavery side. "The views of the Pres: of the Systematic & progressive efforts of certain people of the North, to interfere with & change the domestic institutions of the South, are truthfully & faithfully expressed. The Consequences of their plans & purposes are also clearly set forth, & they must also be aware, that their object is both unlawful & entirely foreign to them & their duty; for which they are irresponsible & unaccountable; & Can only be accomplished by them through the agency of a Civil & Servile war."
Full text here:
http://fair-use.org/robert-e-lee/letter-to-his-wife-on-slavery
Again, reading this letter I see someone supporting slavery as it existed at that time, even though he calls it an "evil." He believes freeing the slaves at that point in time would be a greater evil. "Although the Abolitionist must know this, & must See that he has neither the right or power of operating except by moral means & suasion, & if he means well to the slave, he must not Create angry feelings in the Master; that although he may not approve the mode which it pleases Providence to accomplish its purposes, the result will nevertheless be the same; that the reasons he gives for interference in what he has no Concern, holds good for every kind of interference with our neighbors when we disapprove their Conduct; Still I fear he will persevere in his evil Course."
How many more generations of people, how many millions more people must suffer in bondage? Lee doesn't really care.
"How long their subjugation may be necessary is known & ordered by a wise Merciful Providence. Their emancipation will sooner result from the mild& melting influence of Christianity, than the storms & tempests of fiery Controversy. This influence though slow, is sure. The doctrines & miracles of our Saviour have required nearly two thousand years, to Convert but a small part of the human race, & even among Christian nations, what gross errors still exist! While we see the Course of the final abolition of human Slavery is onward, & we give it the aid of our prayers & all justifiable means in our power, we must leave the progress as well as the result in his hands who sees the end; who Chooses to work by slow influences; & with whom two thousand years are but as a Single day."
So yes, Lee supported slavery, and so did every southerner who joined the confederacy, whether that was what motivated them to join or not, because they all knew what it was about. Certainly you'll see a number who will say they joined because their homeland was being invaded. But they knowingly joined an enitity they knew was perpetuating slavery, so by joining it they accepted it and its goals.