It's true that Illidan is a more ambiguous figure than most raid bosses, but a good guy he ain't. I don't know much about the war of the ancients but he sort of fought for both sides there, earning the epithet "the betrayer." He was a blood elf - that is, addicted to magical power and willing to turn to demonic sources of it - long before there was a group called that. And he was a cruel overlord to the Broken. When Illidan claimed Outland from the demons, I think Akama called it trading one master for another.

As for the black dragons, it's two different groups you're talking about. The nether drakes in Outland were children of Deathwing, but abandoned by him and physically changed by the weird energies in Outland. They weren't black dragons any more. While Sartharion apparently has some kind of story explaining an evil plot behind the eggs, I think it's something like what Nefarian was up to in Blackrock Mountain. But besides, Sartharion is a black dragon, loyal to Deathwing. He's never good news.