Harriet Beecher Stowe's Uncle Tom's Cabin infuriated the South because it

Harriet Beecher Stowe's Uncle Tom's Cabin infuriated the South because it portrayed a black person as the martyr, as an honorable, good, and wise person. whereas a white person was shown as a sinner and a bad man.

As you already know, the South was the part of America that had slaves and that was against the abolition of slavery, so of course this novel infuriated them.