Just checked the list. South Dakota joins Vermont, Maine, and Iowa in having the lowest homicide rate in the US. Minnesota and Idaho close to bottom, too. Trying to find a commonality, I notice all the bottom ten states have short summers, cold winters and a fair amount of open space. Which sort of jibes with social science research I've seen on climate, population distribution, and homicide rates.
DC is well above all the states in homicide rate, which I think might relate to it being entirely densely populated urban and with no suburban and rural areas averaged in to that. States generally are not just consisting of a city. If you included suburban VA and Maryland, I think the picture would change.