I'm still convinced that USC - UCLA is the most purely bitter (not necessarily the best) animosity-filled rivalry in college sports.
For singularly ridiculous and almost unbelievably ill mannered partisanship, this Missouri - Kansas thing may take the cake.
You can have your Ohio State v. Michigan or Alabama v. Auburn, but the last time I checked nobody from Columbus ever went to Ann Arbor and systematically executed every man they could find while burning the town to the ground. And certainly nobody made t-shirts later celebrating that fact.
But that did happen in 1863 in Lawrence, KS when William Quantrill led his band of "Bushwackers" to the "Jayhawker" stronghold and went on a 4 hour rampage that would become known as the "Lawrence Massacre" - one of the ugliest episodes of the brutal 10+ years of fighting along the Kansas and Missouri border. While the Civil War has become the South v. the North in most people's minds, the fighting in fact began as a violent guerrilla conflict between the abolitionists in Kansas and the slave holding Missouri settlers (more or less, like many guerrilla campaigns there were quite blurred lines at times). In many ways, those old wounds have never quite healed - Grandpa Simpson will be be deep in the cold, cold ground before he recognizes Missour-ah as a state, for example.
Those t-shirts seen above that some Missouri fans are making for the showdown at Arrowhead in two weeks are celebrating the Lawrence Massacre and in fact have Quantrill's visage and slogan emblazoned on the back - "Raise the Black Flag and Ride Hard Boys. Our Cause is Just and Our Enemies Many". Talk about going straight past normal levels of fan behavior and making a hard right turn into loony land, that might be the single most offensive gameday t-shirt I've ever seen. Kansas fans are now responding with t-shirts sporting noted violent Kansas abolitionist John Brown (who led a massacre of his own and the 1859 Harper's Ferry raid that really kicked off the Civil War powder keg) with the slogan "Keeping America Safe From Missouri Since 1854" - a mock-up of those t-shirts can be seen here.
On the one hand, this is cool because there's some true blue (ugly) American history linking those towns that host the colleges. On the other hand ... there's so many things wrong with it. So very, very wrong.