It's impossible to be even an average president if you actively support people who are rebelling against the government because they want to have the right to own other people. Do you disagree with this?
Their rebellion wasn't fought purely on the basis of owning people.
Neither was the Confederacy's.
Uh, yeah it was. And don't give me that states' rights crap. The "states' rights" in question were the rights of the states to legalize owning people.
Key here is your use of "purely." I wouldn't deny that it was a contributing factor, but considering that 4 of the 11 states seceded after Lincoln promised not to interfere with Southern slavery, simplistically saying that slavery was the sole reason for secession is intellectually dishonest.