As with your previous question, this one appears to be taken from For the Record, edited by Shi and Mayer. My answer comes from the excerpt of Howell Cobb’s letter that is printed in that volume.
This excerpt is a series of complaints from a Southerner about how poorly the North is treating the South during Reconstruction. It portrays the South as honest victims of vengeful Northerners. It repeatedly says that the Southerners are honest and trustworthy. The author is also very unhappy with the North’s attitude towards the South and their lack of understanding of the Southern position.
Cobb argues that this mistreatment has only one purpose. He argues that the North wants to create a system of black supremacy in the South. He believes that Northerners hate the South so much that they want to do this. Cobb says that the North is willing to do anything, no matter how unconstitutional, to
Make our former slaves our masters, bringing those Southern states under the power of negro supremacy. (The italics are in the text as presented by Shi and Mayer.)
In short, then, the real motivation is to give the freed slaves complete domination of the white Southerners.