Of course, one can very legitimately argue that the Civil War was not about slavery at all. And I'd be the last starry-eyed idealist to suggest that the North had entirely pure motivations in going to war. Politicians are politicians, and on both sides, idealism was pretty much confined to the individual soldiers dying for a cause.
The North's reason for going to war was an interpretation of the Constitution that, right or wrong, considered (as one sees in Lincoln's address) secession to be an act of rebellion against a legal government. I believe Locke would support that interpretation, but more on that later.
However, whatever the North's reasons for going to war, it is very difficult to take seriously any argument advancing the idea that the South was indifferent to the slavery question in seceding. For one thing, if they were not remotely afraid that Lincoln was interested in abolishing slavery (remember, KA-NB act man), why does he spend the first seven paragraphs of his inaugural address reassuring the South that he does not believe he has the legal right to abolish slavery?
Interestingly enough, Lincoln's address seems to suggest initially that the federal government has no right to legislate morality, which makes one wonder what his policy might have been in the current debate. On the other hand, his closing comments actually clarify this original position, suggesting that while he as president does not have the authority to interfere with slavery, he recognizes the right of the union as a whole to ratify amendments to the Constitution that would, logically, include any aiming to do what most of the Founders had wanted to do in the first place: make slavery as taboo in political law as it is in moral law.
For reference, here's the quote, though it would behoove anyone interested to actually read the address and note the movement he makes over its course:
This country, with its institutions, belongs to the people who inhabit it. Whenever they shall grow weary of the existing Government, they can exercise their constitutional right of amending it or their revolutionary right to dismember or overthrow it. I can not be ignorant of the fact that many worthy and patriotic citizens are desirous of having the National Constitution amended. While I make no recommendation of amendments, I fully recognize the rightful authority of the people over the whole subject, to be exercised in either of the modes prescribed in the instrument itself; and I should, under existing circumstances, favor rather than oppose a fair opportunity being afforded the people to act upon it. I will venture to add that to me the convention mode seems preferable, in that it allows amendments to originate with the people themselves, instead of only permitting them to take or reject propositions originated by others, not especially chosen for the purpose, and which might not be precisely such as they would wish to either accept or refuse. I understand a proposed amendment to the Constitution—which amendment, however, I have not seen—has passed Congress, to the effect that the Federal Government shall never interfere with the domestic institutions of the States, including that of persons held to service. To avoid misconstruction of what I have said, I depart from my purpose not to speak of particular amendments so far as to say that, holding such a provision to now be implied constitutional law, I have no objection to its being made express and irrevocable.
The point is, Lincoln reminds the South that he is not attempting to overstep his own legal limitations and abolish slavery. Yet the interpretation of the Constitution that he presents also reminds them, implicitly, that they are as bound by the Constitution as he is, and must be willing to adhere to that Constitution whether it abolish slavery or no.
On the other hand, the South, was attempting to counter what they assumed to be necessarily unconstitutional--that is, the federal government finally confirming that human beings are not property, nor can they be in any legal sense so. In objecting to Lincoln in particular, they were, in the first place, presupposing an unconstitutional act on the president's part that had not taken place; and surely one can't justify secession (again, I'm assuming Lockean terms for the purposes of this post) based on the idea that the government might do something illegal. The underlying, and probably much more deeply-rooted objection was to the idea that the government can make any laws at all regarding morality. In a democracy, is principle to count at all (as Lincoln had argued in the KA-NB affair), or is legality simply a matter of majority rule?
If the latter, I think we all need to have a problem with trying to outlaw abortion. Don't want to start getting ahead of ourselves here. I mean, we couldn't actually have the federal government confirm basic ideas like "Thou shalt not steal" and "Thou shalt not kill", could we?