Dai, technically, Greg is correct- No constitutionally opposed states were in the Confederacy or v. v., but that doesn't mean that their citizens were united in their beliefs.

Mob action including murder and arson between whites was common in Missouri, Kansas and elsewhere over the issues. The state of West Virginia was split off from Virginia to add one more to the Union side and to exert control in an area that mostly, but not wholly, supported the Union.

Beliefs are hard to change and they persist. My wife's relatives, Southerners all, still say that the War Between the States was about states rights, and had little to do with slavery.
My relatives were all on the Union side- but were they fighting to end slavery? Mostly they were fighting to preserve the Union and didn't worry overmuch about slavery.

If you read the essays on the Negro in the Encyclopaedia Britannica as late as the 11th edition (ca. 1910), you will find that people were still taught that the Negro was an inferior being, with all the stereotypes discussed. These attitudes were common in the Union states (and the English-speaking world) as well as in the Confederacy.