What it all boils down to is this: did the South have every right to secede from the Union? Absolutely! The Constitution does not forbid states to secede. Arguing over whether or not it was about slavery is a moot point as far as I'm concerned. Lincoln and his congressional cronies had no authority to prevent the Southern states-- states that had voluntarily joined the Union under the premise of reassuming their soverignty should the need arise-- from willingly leaving the Union.
So, given that the states had every right to secede and Lincoln had no authority to stop them, he successfully trashed the Constitution. In the grand scheme of things it doesn't matter what he thought of slavery. He used his position of power to further his own vision of what the "Union" should be. Need we point out that the secession of the South would not have destroyed the Union? The United States would have continued to exist, albeit with 13 or so fewer states.
From a constitutional standpoint Lincoln was wrong to invade the South, and that fact alone makes him a bad president. The only way anyone can believe he was right would be to read into the Constitution something that simply isn't there. No one can deny that since 1865 we have seen more powers taken from the states and given to the "federal" government. (Actually, it's more of a national government now.) Lincoln's personal war was one of the worst crimes ever perpetrated on the people of this nation.