The south didn't fight to keep slaves, they seceeded or as you say rebelled (your words) to keep slaves. The union and lincoln insisted on fighting a war. That was why we fought a war. (simple proof: if the south had stayed in the union there would have been no war).
When I say the rebellion was moral, I am just repeating what I have learned from many of the unionists on this board. According to the Declaration oppressed people have the right to rebel. Do you disagree with that. If so, then you disagree with the american revolution.
Finally, my actual opinion is the south's separation was a legal secession. You insisted it was a rebellion. You may want to switch your opinion to secession as it may be an easier argument to claim it was illegal and thus immoral.