Well, Ulfric did give Toryyg the chance to join his side or at least speak out to what he was saying. His followers say he would've joined his side if Ulfric asked, but Ulfric couldn't possibly know that. All he knew of Toryyg was that he was the Empire's posterboy, and that he was weakening his beloved country by suckling the Empire's teets and allowing Thalmor to do what they will. If you ask me, Toryyg got plenty of chances, and Ulfric was done waiting. I would've done the same thing.
He's a man of action. He's not hasty though, as his patience with Whiterun and Balgruuf will show. You can tell from his dialogue with Galmar that he's a patient man, which really makes me think Toryyg really didn't show any interest to Ulfric's cause at all. I think the young king brought it on himself, personally.
Yes, Toryyg had zero chance against Ulfric, thu'um or no thu'um, but that is irrelevant. You gotta look at it from a nord perspective. He followed the ancient law, and Toryyg accepted. Toryyg, as a king under the Empire could have said no. People said that would be a political suicide, but if you look at the people on his side, and the Empires, Toryyg would have had more if a moot was called, and Ulfric still would have lost, and Toryyg would remain King.
Even if you think what Ulfric did was wrong, to me that doesn't matter. No country becomes free without breaking a few necessary eggs. You do what you must. War is not pretty,