Many people disapproved of the Kansas-Nebraska Act because it repealed the Missouri Compromise.
The Kansas-Nebraska Act was the law promulgated in the United States, in 1854, for the creation of the states of Nebraska and Kansas, in territories of the old French Louisiana, but in which remained some tribes of Native Indians. It was driven by the senator and leader of the Democratic party Stephen Arnold Douglas, of Illinois.
The situation of the two states north of the line defined in the Missouri Compromise meant that both should be states in which slavery was not allowed. However, the contiguity of Kansas with the slave state of Missouri and the search by Senator Douglas for southern support for a railroad in his state caused the law to include the provision that, in order to decide on the issue of slaves, citizens could exercise "popular sovereignty" and, therefore, be able to decide whether to be a slave state or not.
The discussion of the law and subsequent voting provoked strong conflicts between anti-slavery and pro-slavery parties, especially in Kansas, and the disappearance of the Whig Party (divided between supporters of the law in the South and those opposed to it in the North), and the creation of the Republican Party.
The Kansas-Nebraska Act effectively nullified the Missouri Compromise of 1820, and although it failed to make Kansas a state with legal slave labor, it further opened up the nation's divisions that led to the Civil War in 1861.