Not sure what "horrible" refers to...
As for slavery, it was mentioned at great length in the original "Reasons for Secesdion" documents.
It was then added specifically to the new Confederate Constitution.
Slavery has also been of great importance to the Union in the form of "Contraband of War", which will, in due time lead Congress to act & Lincoln to his Emancipation Proclamation.
And if you were hoping to see Union troops marching off to war singing "Come on Lads, let's go free some slaves"...
Well, it had already begun, sort of -- a catchy tune with the words "John Brown's body lies amouldering in the grave.." is already being sung by some Union regiments, and in due time a certain poetical lady will find even more inspiring words for it.
Regardless, your basic observation is correct: Civil War was first and foremost all about preserving the Union, secondarily about abolishing slavery.
Isn’t it strange, then, but after four long horrible years of war, with hundreds of thousands of dead, the union did not begin to free the slaves until after the war was over.