The Kansas-Nebraska Act repealed the Missouri Compromise that had settled relations somewhat between the North and the South over the issue of slavery. The Act allowed slavery in the two territories based on referendum. It was assumed that Nebraska would become a free territory and Kansas would become a slave territory. Neither territory would become a slave territory after violent fighting over the issue.
The act greatly angered the North which felt betrayed. Previously there was a line that dictated whether territories would be free or slave states. This new law allowed territories north of that line to become slave states by a vote. Northerners chose to disregard the Fugitive Slave Act in retaliation, which angered the South. The Republican Party was founded as a result of this law. At first the South saw this law as a political win, but the law had consequences in the North (Republican Party formation and the election of Abraham Lincoln) that angered the South enough to secede from the Union.