According to Dale E Watts of the Kansas State Historical Society, "Bleeding Kansas" was not nearly as violent as once thought. (See article in PDF here) For those unfamiliar with the period, "Bleeding Kansas" is the name for the territorial period of 1854-1861 when Kansans fought over whether the state would enter the Union as a slave state or a free state.
Contemporary accounts wildly exaggerated the bloodshed. Watts attributes this to the fact that after the abolitionists won the conflict, they reported much more killing by pro-slavery men then had actually occurred in order to highlight the righteousness of their own cause.
Instead of the street running red with blood as comtemporary abolitionists would have had us believe, Watts reports that during the seven years as a territory, only 157 violent deaths are found (22.4 per year). Only 56 of these were due to slavery-related killings (8 per year). So, if we take out the slavery related killings, the entire vast territory of Kansas experienced approximately 14.4 violent deaths per year.
"Bleeding Kansas" Not so Bloody After All
