To take them in order, the Fugitive Slave Act angered many people in the North because of the prospect of official slave-catchers hunting down runaways and returning them to their owners. ('Uncle Tom's Cabin' was born out of this protest.)
The Kansas-Nebraska Act looked like a sensible plan to allow the people of each state to vote whether to be slave or free. But it just encouraged terrorists from both sides to intmidate voters and try to cause confusion by declaring all results to be rigged. This violence was like the upcoming Civil War in microcosm.