It is unlikely that Kansas would have become a slave state (Kansas approved its free state constitution by a 2-1 margin), and even if it did it would not have seceded.
Support for secession was directly tied to the proportion of slavery within the state. Thus, in the Deep South dominated by slave based plantations, the states quickly seceded. In the Upper South, where slavery was widespread, but not to the extent of the Cotton Belt, the states eventually seceded once it was obvious Lincoln was sending troops. In the Border States which had some slavery, but whose economies were not based on it, none seceded. In those areas of seceded states where slavery was not dominant (mainly the Appalachia country), the sentiment was always pro-Union and anti-Confederate.
A slave state Kansas would not be dominated by slavepower interests. The slave holders would not be able to force through secession. Kansas did not have a geography conducive to plantation economies. Only a small area near the Missouri River was viable for it. Kansas will remain majority free with a distinct slavery minority. The only way slavery would be approved is becauseof a willingness to compromise and make peace with a minority of the population, and the slaveholders would need to come up with something else in exchange. The slaveholders would never dominate the state.
The overwhelming population will remain loyal to the union and fight for it. In fact, in this scenario, I would not be surprised if within 1-2 years Kansas would approve a new constitution to abolish slavery.
Kansas is way too isolated from the basis of Confederate power to play any role in the civil war. The Confederates couldn't even keep the trans-Mississippi open IOTL. The Union will still control the Mississippi River relatively soon and cut off the Confederate west. They won't be able to project power. Some bushwacker guerilla groups may operate, but they'll quickly be eliminated in Kansas. The war pretty much happens as it does IOTL.