Why would some northerners have been upset over the passage of the Kansas-Nebraska Act?

Northerners were mad because residents in Kansas were allowed to use popular sovereignty (they were allowed to vote and decide to allow or stop slavery in Kansas) and this meant the 36-30 line was repealed or erased (remember that the 36-30 line was used in the Missouri Compromise of 1820 and that no slavery was supposed to be allowed in places like Kansas that were above or on the north side of the line).