The winners write history so as to place their "side" in the most favorable light. Slavery was only part of why the Southern States formed the Confederacy and seceded. Yet today it has become the whole reason for the separation.
The truth is that Northern States had a greater population than the Southern States and they had more seats to vote with in the House of Representatives. As a result the Northern States whose economies were based more in manufacturing had little understanding of the mostly agricultural economics of the Southern States.
Northern politicians could not see that the laws that they wanted to force upon the Southern States would send the South into an economic crisis. Also Southern States felt that the rights of the States should always override any federal laws. This State loyalty could readily be seen by the military units that fought in the Civil War. The 1st NC, the 2nd Texas, the 5th Alabama, or the 3rd VA were common military units that State Governors supplied to fight for the Confederacy. The Northern States did the same.
Lincoln did not sign the Emancipation Proclamation at the breakout of war with the South, but only after suffering repeated military defeats and he greatly needed the blacks to join and serve in the Union Army.
This is just some of what gets swept to the side when the winning side writes the history.
You should hear the British talk about how they won WW2 and the how Americans helped as little bit.