This is a complicated issue. My opinion is that he is not a murderer, HOWEVER the use of the shout was NOT fair combat, at least not by most definitions.
Why he isn't a murderer:
1) Torygg had the choice to say no. It's not much of a choice, but Torygg went into the fight knowing that he would likely lose, and was willing to die rather than give up his throne.
2) It was a duel sanctioned by Nord tradition.
Spoiler 3) If you go to Sovdgarde during the main quest and speak with Torygg, he says that he does not regret his decision to fight Ulfric, knowing he was going to die. Torygg is happy that he died with honor and in battle and therefore can go to Sovngarde.
Why this wasn't fair combat:
1) By most definitions that I have read, fair combat involves both sides using the same weapons. Ulfric had a weapon that Torygg did not - the shout, and used it, which in my opinion is dishonorable and not in the spirit of fair combat.
2) Several people have stated that Torygg could have easily just learned to shout like Ulfric did. This is NOT the case. Ulfric's situation was unique. He was called to study with the Greybeards at around 10 years old and dedicated the next DECADE to learning to shout along with them. You don't just waltz up to High Hrothgar, spend a lovely weekend there, and leaving knowing how to fus roh dah people to death. Ulfric had to dedicate a decade of his life to the study. In order to shout equally, Torygg would have had to go to High Hrothgar as a child and spend 10 years learning how to shout. It's just an unreasonable expectation for almost any Nord to do this.
3) Torygg didn't have much of a choice in whether to fight or not. I mean, he sort of did, but if he said no he would be seen as a disgrace and unfit to hold the throne, and having dishonored his family.
My conclusion is that Ulfric did NOT murder Torygg, but his method of killing him was dishonorable and wrong.
As for the issue with the Way of the Voice, I honestly don't think that the fact that Ulfric doesn't follow it is criminal. It is his decision. He was taken to High Hrothgar as a young boy and not really given the option to decide if the Way of the Voice was what he believed. He left High Hrothgar when he realized he could not follow that philosophy despite the Greybeards' attempts to indoctrinate him. That is not a crime. The Way of the Voice is a choice, not the law. Despite his leaving, Ulfric himself states that he uses the Voice sparingly and holds high respect for it and the Greybeards, even though he failed to follow the way like they did.
And finally, as for the claim that Torygg might have tried for independence if Ulfric had asked... I doubt it. Torygg was extremely impressionable as a King. He was surrounded by Empire-favored advisors who told him what to do and how to act (the same ones who do the same not for Elisif, unfortunately). Of course Torygg would consider independence if Ulfric asked - Torygg considered ANYTHING ANYONE told him, because he didn't have the experience to make his own choices. But I can guarantee that the moment Torygg brought what Ulfric said up with his "advisors," they would stop any notion of succeeding from the Empire real quick. While I don't think that Torygg deserved to die, Ulfric's claim that he was a puppet of the Empire is not far from the truth.