Although John Calhoun was a staunch supporter of slavery, he also wanted the Union to remain intact. His biggest concern about abolitionism was that it would cause so much hatred between the North and the South that the country would inevitably come into conflict. In other words, Calhoun feared that abolitionism would bring on a civil war.
Calhoun believed that slavery was a positive good. He argued that it was a better system than the system in the North or anywhere else with free workers. He points out that such workers were not cared for while sick or old the way slaves were.
But Calhoun wanted the Union to stay together and he believed that abolitionism would prevent this from happening. He believed that it would cause the North and the South to come to hate one another. He says that abolitionists
... hate the people and institutions of nearly one-half of this Union, with a hatred more deadly than one hostile nation ever entertained towards another.
He believes that this hatred will cause a split between North and South. He says that unless abolitionism
... be speedily stopped, it will spread and work upwards till it brings the two great sections of the Union into deadly conflict.
This was his great fear. He feared that abolitionism would bring a civil war about.