That may have been a factor or even the main motivator for some people, but I don't know of documentation to support that as a primary argument for Lincoln. Lincoln wrote and spoke extensively about his legal arguments. Much of it was tied into his own constitutional oath and his obligation to ensure a Republican form of government. Re the oath-- he reasoned that if states could secede, then they could secede and secede and secede, and eventually there would/could be no federal government and no need for the federal Constitution. Therefore, to uphold the Constitution's very existence, he concluded no state could secede. Second, re the Republican government clause, he reasoned that the only way to ensure all "current" citizens had a republican form of government was to prevent any place wherein a federal citizen lived (i.e., any state) from seceding because, once the state had seceded, the federal government would no longer be able to control the form of government in that state.

He had some other similar legal arguments, but they were centered around the Constitutional issue of whether or not a state could secede. Lincoln did not "free the slaves" in the Civil War or engage in a war solely to end slavery. The non-seceding states kept their slaves until the Reconstruction amendments because the Emancipation Proclamation was a tool used to deprive the ENEMY (i.e., seceded states) of a resource--slaves. States that didn't secede, however, retained their slaves. Thus, ironically, the Reconstruction amendments were necessary to end slavery because Constitutional amendments were necessary to amend the Constitution. The South was right on that point, at a minimum.

There's a whole lot more... but that's a little more depth on the side note...

It's not flattery if it's true.