The Ebon Dragon doesn't have a Motivation, but he has an Urge: "Darken all of existence until Virtue and light cease to be, leaving blackened chaos upon which his will alone dictates possibility."
How does that work? Well, in Creation, Virtue anchors existence. The prime mover is shinma Advaita Iraivan, God, an aspect of Nirguna. (Don't talk to me about the shinma of separation. That is, if anything, Niruphadika, the Way.) Nirguna does not exist, and neither does shinma Advaita Iraivan, but they nevertheless support existence and Virtue through ties to Dharma, Nirvikalpa, Nirvishesha, and Nishkriya. By destroying or banishing Virtue, one destroys the influence of the shinma the Primordials used to strengthen Creation's ties to existence.
But the Ebon Dragon isn't a nihilist. He doesn't want to destroy everything. He wants to control everything. "…leaving blackened chaos upon which his will alone dictates possibility." He doesn't want to destroy existence, he wants to define it.
The Ebon Dragon's goal isn't meaningless wickedness. The destruction of virtue is a methodology. His goal isn't to become the Ultimate Darkness. It's not to stomp your hopes into the ground.
His goal is to supplant Nirguna and become God.
(He's qualified for the position. Nirguna doesn't exist, and as an aspect of Nirguna, shinma Advaita Iraivan, God, doesn't exist. But the Ebon Dragon doesn't exist either. He is a himself-shaped hole where he should be.)
The Ebon Dragon can't abide his own restraint. To the extent that, regardless of your personal definition of "Good," it's going to boil down to "How we should behave," he's evil because he can't abide you telling him how he should behave. He will not do what you think he should do, because that's you dictating terms to him. This does not, necessarily, mean he hates you or wants you to suffer or cares about you or even thinks about you in the slightest, except at one step removed.
If Nirguna defines existence, he can't abide that because that's something other than him dictating to him the terms by which he exists. He wants to become God because then he will dictate his own terms. Remember that when restrained he has a dice pool of 0 for any actions not dedicated, ultimately, to freeing himself. He can't do anything other than this.
"The villain who wants to become God" is a common fantasy archetype, so clearly Exalted needs one of those. Where the Ebon Dragon differs from the typical JRPG antagonist is his methodology of doing so is actually quite logically constructed within the setting's larger metaphysical structure, and also sufficiently obscured within that structure that "Villain who wants to become God" is not the only or even the first thing you will notice about him when you look at him.
His evil, such as it is, is largely incidental to his atavistic pursuit of his own freedom — or, it would be, if Creation's metaphysics didn't make "be evil and promote evil to the extent that it is possible to be evil and promote evil" a valid path to Godhood for a being that doesn't exist.
The Ebon Dragon as ultimate-nemesis-for-the-sake-of-ultimate-nemesis is annoyingly caricatured, and while Neph may think it's a good idea to make the Ebon Dragon completely one-dimensional for the sake of making a philosophical point about the nature of evil, I can't agree. The Ebon Dragon as ultimate-nemesis-because-it-gets-him-what-he-wants is, I feel, slightly more nuanced and useful.
(This somewhat more nuanced view of what the Ebon Dragon is about is also really helpful for figuring out how he's not Szoreny.)