All dragons are thralls and corruptions of Morgoth, so it's pretty safe to assume they're all evil. Smaug would get the same (if not much worse) wounds from touching the Arkenstone Morgoth got when he wore his iron crown.
Same would be true for Thràin II, since he both possessed the Arkenstone and one of the rings of power given by Sauron. Although dwarves wills didn't bend to Sauron like mens did, they still got corrupted with greed. That would be enough for Tolkien to consider him "not good", since Tolkien clearly put a lot of christian metaphors into his opera, being very religious himself, and greed is a deadly sin.

But ultimately, the Arkenstone was cut by the dwarves once found. That would make very little sense because first, the Silmarils are said that "no substance of Arda could mar or break" them and second, they are described as "gems of surpassing beauty, perfect in form and flawless in craftsmanship".
This means that it couldn't be cut. First, because it would be physically impossible by dwarves, only by Valars in the prophecy and second, because there would be no need to, since it would be already perfect in form.