The Elder Gods drove him nuts, basically. He's the Earth Aspect (like Malygos is the Magic Aspect, Alexstraza is the Life Aspect, etc), thus he's kinda the avatar of physical existence. Since he's been driven mad, he now reasons that nothing at all should exist without his say-so. This has eventually given way to 'nothing at all should exist'. He previously tried to enslave all the other Dragon Aspects in order to re-write existence itself into a form he preferred. It didn't work out so well. So now he's gonna kill everyone who would get in the way and then take his time recreating things.