I've read that a higher percentage of northerners owned slaves than southerners. I'm not sure if that's true, and it would be hard to verify in a history book because it's not something that historians would want to publicize. The argument I read was that while there were a larger number of slaves in the South, they were owned by very few wealthy people that owned large plantations. Most people in the South were too poor to own slaves. In the North there was a larger number of wealthy people that owned slaves to work in their household. I'm sure Norther slaves were treated better; but if it's true, I find it ironic that the South gets such a bad rap regarding slavery.
I've also read that Lincoln won the election despite not even being on the ballot in most Southern states. I firmly believe that the slavery issue was the major reason for succession and was caused by rich Southern landowners; but I have to believe the vast majority of the hundreds of thousands of soldiers that fought in the war, did so for other reasons than the interest of a few plantation owners.