With Tolkien, ultimate destruction was not so much the point as was control of Arda. Melkor never understood the extent to which Eru could fold his acts into His greater plan of creation. What he tried to do was similar to an adolescent messing up his bedroom to show mom that she isn't Big Boss anymore. He was trying to say he was his own man, and his own ideas, orders be damned.

And if you question what he would do after he won, shouldn't the same be asked of Manwe? After Sauron's defeat, there was no threat that would warrant his attention. Even Sauron was easily handled, if you take into account the sheer power of the Valar. Wouldn't it be more natural, more easy to understand, if Manwe allowed Dark Lords to keep flourishing now and then to keep himself and the Valar entertained?

But we accept that his motivation of ultimate Good is sound, because that appeals to our outlook. Perhaps Morgoth, with his "independence" and "non-conformity to authority" would appeal to people with other ideas of how the world should be run.

With the Creator and the Dark One, I always felt Good and Evil were adjectives applied to them because this was the simplest way for human beings to understand such alien powers whose goals and methods are beyond normal human comprehension.

Frankly, I wouldn't be surprised if what the Dark One objects to is not Creation, but the order and structure that it has (him being called the Lord of Chaos and all). It could be that the Pattern and the Wheel enforce structure on the Universe that is anathema to the existence of the Dark One. His desperation to disrupt this order and impose an environment conducive to him would then be similar to how we would attempt to rid a room of Carbon Monoxide before entering and inhabiting it.

Either way, it is not clear at all what Rand has to do to defeat the Dark One. Killing him is clearly not an option, based on what RJ said. Maybe some kind of compromise?

RJ once said that the concept of circular time did not appeal to him at all, that it represented a kind of stasis (echoing what Moridin later thinks). Maybe the series will end with the structured repetition of the Ages ended once and for all, the grater unpredictability of the new reality causing the DO to stop attempting to destroy it. Ishamael should be happy with this result too.