On one hand you argue that it was to end slavery - since the south seceding would have continued it you assert that the union had the right to prevent it, yet if the confederacy had remained in the union, there was no power to end it. In fact, there would have to be 52 states in the Union today to end it via amendment (to counter the 13 Confederate states). If the south invaded Rhode Island & Providence Plantations, Connecticutt, Massachusetts or New York to end the slave trade, would that be justified?
Slavery was happening in America, and that is unacceptable.
The framers protected it and allowed it to perpetuate. The founders, when declaring their independence from Britain also possesed slaves, but I have yet to see a single Lincolnite attempting to denigrate that separation.
Some Founders owned slaves and that was wrong. The American Revolution, however, was not begun over slavery, so I fail to see the connection. It would be unfair to say that they all perpetuated it, however, as there were many Founders who were against the practice.
For that matter, Robert E. Lee found slavery personally deplorable. One of the most moving tragedies of American history is, in my opinion, that a thoroughly decent, brilliant, and God-fearing General fought for the wrong side. I understand his devotion to his state, but that doesn't change the fact of history.
Incidentally, this is why I will be first in line at Gods and Generals.