I think he's making a point about the thu'um and in general the Nords, that they've grown weak and dependent on the empire. When you ask him about learning to shout, he says that any Nord can learn if they have the disicipline and will- "Torygg had neither, and that was the point." He says Torygg was not a true high king of Skyrim, "his father maybe, but not Torygg."
Despite that, I don't get any sense that he hated him. He just saw him as a puppet of the empire, and in the end, a necessary sacrifice.
This is right. Ulfric did not hate Torygg, only what he represented, and felt that this was the onyl way to deal with it (which I disagree with, but whatever).
The High King is said to have looked up to Ulfric. I've heard it suggested that Torryg would have done something against the Empire if Ulfric had asked, but instead Ulfric decided to kill him and usurp the throne. I would bet Ulfric didn't even need to use thu'um to overtake Torryg either, but did it anyway because he likes being extreme.
These things that you "heard" come from Sybille Stentor, one of Torygg's advisors. She says that if Ulfric had asked him to succeed from the Empire, Torygg MIGHT have done it. Maybe. At least, he might have brought it up with his court, right before his gaggle of Empire-appointed advisors shut down the idea and he changed his mind again. Torygg, like Elisif, was extreme susceptible to suggestion because he was young and inexperienced, a fact that the Empire took advantage of (that is why Elisif has about 3 times as many advisors as any other Jarl). Pretty much anything anyone told him sounded good to him. But if you think his advisors would have let him succeed from Skyrim, I think you're pretty naiive. They won't even let Elisif send soldiers to a freaking cave. They have her locked down tight. A pity, since she is very intelligent and kind and could be a good leader someday.
Ulfric admits that he could have killed Torygg without the Voice. For him, it was about creating an image of a powerful man who could literally shout away the evil forces of the world. Pretty dishonorable, in my opinion, to use the Voice, but he did.