The reason for any Mizzou fan to hate Kansas is simple: it's in our blood. In the 1850s, when Kansas was slowly becoming a state, the federal government was giving new states the option of coming in as either a free state (Union/North) or a slave state (Confederate/South). Missouri had entered in as a slave state, and Kansas was wanting to enter in as a free state. That's when the trouble started. Years before the Civil War even started, there were outbursts of violence and battles between the two states that became known as "Border Wars". Members of the Kansas state guard, known as Jayhawkers, invaded Missouri and pillaged the towns along the border. Members of William Clarke Quantrill's Raiders did the same thing from Missouri to Kansas. This went back and forth for years, until September 23, 1861, when Kansas senator and colonel in the state guard James H. Lane invaded Missouri, sacking and burning the towns of Butler, Higginsville, Clinton and Osceola in an attempt to push out pro-Southern forces. Lane and his Jayhawkers killed indiscriminately...men, women and children were all slaughtered in cold blood.
Quantrill, seizing an opportunity for eye for an eye, moved west with his Raiders into Lawrence, hometown of Lane. On August 21, 1863, Quantrill led a group of 450 guerrillas into Lawrence in an event that became known as the Lawrence Massacre. Quantrill's Raiders burned the town to the ground in retaliation of Lane's actions in 1861. The only ones killed in the raid were men and boys that were "old enough to carry a rifle". Senator Lane escaped in his nightshirt. Kansas returned the favor by depopulating four Missouri counties, burning building after building and killing anyone that got in their way.
So how does this affect us today? It's about bad blood. Missouri and Kansas have never gotten along, as proof has been afforded above. The real rivalry started on the battlefield, and has since transferred over to the sporting venues. For the first part of the Missouri-Kansas football rivalry (second oldest in the NCAA behind Minnesota-Wisconsin), games were played in Kansas City, due to neither team feeling safe in the other team's town. The two have even more of a history as they played in the first ever Homecoming football game on November 25, 1911. Then Mizzou Chancellor Chester Brewer called for all Mizzou alum to "Come home" for the season finale against the Jayhawks. The game ended with a 3-3 tie. In 1960, Missouri entered the KU game undefeated, and looking at a chance to play for a national championship. Kansas upset then no-1 Missouri with the help of Bert Coan, who was later determined by the NCAA to be ineligible. That game spilled over onto the basketball court in 1961, when the two teams met in what has become known as the Basketbrawl, you can view it below:
The basketball rivalry is anything but, with Kansas leading now 170-69 in the series, with the last Mizzou win over KU in 2009 when the Tigers eventually went on to win the Big 12 Championship. However, the football rivalry is closely watched and is highly contested. KU fans will say the series is now tied, 55-55-9, whereas Mizzou fans, with the argument of Bert Coan on their side, say the rivalry is 56-54-9 Mizzou. The Border Wars goes beyond those two sports, as it encompasses every sport that the two play each other in. Whichever team does a better job in athletics for that year, wins the Border Wars trophy that travels between the schools. The Tigers have controlled the Border Wars, with the exception of men's basketball, for six out of the last eight years.
And besides, when you're born in Missouri, you're given the option: either root for the Tigers, or be one of "them".