This argument that just because someone was elected he isn't a dictator is facetious at best, esp. when it was with virtually half the country clamoring for secession.
Now, I'm not saying Lincoln was a dictator. I am, however, saying that your above logic is flawed.
Where in the world do you get this assumption that slaves would have "inevitably rebelled"? Just from an economic standpoint, slavery was not a sustainable enterprise, and countries other than the U.S. realized this quickly enough. Spain, England, France, Denmark, etc. all were able to end slavery peacefully, without the need for any civil wars, due to this underlying economic truth. The majority of these countries, in fact, peacefully abolished slavery much earlier than the U.S. did. Refer to the following timeline for reference:
http://en.wikipedia.org/wiki/Abolition_of_slavery_timeline
So yes, the purpose of the Civil War didn't lie in freeing the slaves, but to destroy the secessionist movement (or, as you'd prefer to call it, to "preserve the Union"). Never mind that over 600,000 people (or the proportionate equivalent of 5-6 million people today) had to die.
In spite of what you'd like to believe, what remained is NOT the Union as it originally existed, because the Union was voluntarily formed. After the Civil War, this Union was proven to be no longer voluntary. The Civil War wasn't exactly an example of the US being the "beacon and defender of freedom" as you'd fondly like to spin it.