You misunderstand the attitude of the South. They knew it was evil... it was just a necessary evil for their way of life. They didn't think it was a good thing that should be spread everywhere. They wanted new territories and states to be slave states not so the "good" of slavery could be extended but so they would not be crushed by an increasingly abolitionist Congress. They were trying to maintain a balance of power not argue for slavery's virtues.
In 1800, true. By 1850 and even more 1860, not true.
The VP of the CSA, in his most famous speech.
Jefferson in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. ... The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away. ...
Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the government built upon it fell when the "storm came and the wind blew."...
Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner- stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition...
The substratum of our society is made of the material fitted by nature for it, and by experience we know that it is best, not only for the superior, but for the inferior race, that it should be so. It is, indeed, in conformity with the ordinance of the Creator.