Quote Originally Posted by snoopy13a View Post
Thorin is extremely greedy. He is willing to go to war with the men and elves and will not give them a share of the treasure despite the fact that the men killed Smaug and had their town destroyed. Bard was willing to accept only 1/12 of the dragon horde and would compensate the elves from that. Luckily for Thorin, he repents of his greed on his deathbed.
The Arkenstone, specifically, appeared to affect dwarves the way that the Nauglamir with the Silmaril did - driving them to violent greed. (Although the results were even worse in the case of the Nauglamir; but then the Silmarils are the greatest treasures in the history of Middle-Earth.) But it may just be their general greed for treasure.

Really, despite the simplistic black-and-white morality (literally, unfortunately) of LOTR itself, the races of Middle-Earth as portrayed by Tolkien are all variable, capable of both good and evil, and overall "neutral."