It's important to remember that Lincoln did not actually start out wanting to free the slaves. The south saw the writing on the wall as abolitionism slowly grew ever more popular and decided to rebel before it was strong enough to put them down by force.
They were too late.
Basically the south saw their norm losing and flipped out and tried everything to keep themselves in power... and when all of that failed tried to flip the table.
The thing is that the idea that the North wanted to free the slaves was just Southern paranoia, all the vast majority of the North wanted was for the South to stop trying to force them to participate in Slavery. Because by the 1860's the perception existed - and it was a completely justified perception - that the South was going to use their dominance in the U.S. Government to shove Slavery down everyone's throats. To make it a National institution, not just a Southern one.