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.
Frankly I find highly suspect the notion that somehow slavery would have "died off" in a "short" period of time if Lincoln would have just "let things take their course."
Think what you will, but that does not change the fact that it did end peacefully in numerous countries, almost always coinciding with industrialization.
A feeling of inherent superiority amongst Southern whites was deeply ingrained.
No more so that that held by Northern whites. The world was full of racial bigots at that time, including blacks that considered themselves above the poor "white trash" in the South.