Yes there was slavery-related violence going on in Kansas from 1855 on. The Missouri Compromise was 1820 and was a major element in the story. The direct cause of the current troubles was the Kansas-Nebraska Act of 1854, which effectively repealed the Missouri Compromise and made Kansas the focal point in the argument of whether slavery could be extended from where it existed in the southern states (including Missouri) to newly created federal territories.
I never much kept up with the slavery debate for the territories, but in the last year I have learned of the existence of Article IV Section 2, and as a result I no longer understand why there could have been any debate about it.
With the constitution explicitly guarantying that slaves must be returned to their masters, how does one realistically abolish slavery in the territories, or even in "free" states?
How do you stop slave owners from going into the territories or free states with their slaves?