Why was the issue of slavery in the United States so divisive?

The issue of slavery divided the North and the South. The South insisted on slavery, as it needed cheap labor for its production, such as of cotton, which would not be profitable without slavery. The north wanted to abolish slavery, as is it brutal and goes against human rights. Especially the influence of religion and female activists in the north contributed to the negative sentiments towards slavery. However, racism was also widespread in the North, and for many the main opposition to slavery came from the division of South and North, disapproval for slavery was a distinctive feature of the North as opposed to the South, and they placed more attention towards weakening the South (through the abolition of slavery, which was seen as a symbol for the division) rather than actually caring about the lot of slaves.