He used a show of force and power, to prove that he would make a more effective and powerful King. That he would do the hard things, and not bow to Imperial coin. The entire point of the tradition he called. No murder in it. The fact its considered such is an example of how the Empire has groudn down and destroyed much of the Nord culture. Whether you think its nice or not is a completely different topic.
No, it shows that he is willing to bowl over anyone who is in his way. No appeal to Torygg before killing him, just straight to murder by Thu'um abuse. If Torygg was truly such a wuss in a fight like everyone claims, why did Ulfric have to Shout?
If I recall, Ulfric also says the Shout is not something to be used casually (or something to that effect). Sorry, but actions speak louder than words... wait... thu'um IS words.... dangit... you know what I mean.
He also 'abused' the powers of the Greybeards long before this; he was a vetran soldier for the Empire if you'll remember. He left to support the Empire intiailly. He didn't break his oaths to them to form the Stormcloaks. IIRC, he used the voice in 'service' to the Empire, at Markarth. Torygg was the last known time he used his voice. Honestly the Greybeards lack of scolding on Hight Hrothgar should be telling. Its in the past, and while I respect their ideals, I feel its down to the individual to decide how to use the power in their grasp. He has it, he has aprted ways with the Greybeards, and he can use it for the good of Skyrim. In my eyes thats better than pretending he never learned the abilities. They give the Dragonborn a pass on prophetic reasons; but in practice there is very little difference between Ulfric using what he learned and the Dragonborn. Especially since the Latter can be a member of the Dark Brotherhood and absolute monster. But its okay for him to 'Yol' an old lady. The rules don't apply to heroes.
In practice, there is a great big difference: Ulfric learned the Thu'um from men. The Dragonborn's is innate and divinely granted. Ulfric's Thu'um is learned, a tool. The Dragonborn's is just as much a part of them as their heart or mind.
Nice opinion. However, lets look at some historcal facts. The Thalmors main invasion force was destroyed too. Neither side looked healthy at the end, and the terms the Emperor agreed to are supposedly very close to what was initially demanded. Second, the Thalmor don't want Skyrim to win anymore than they want the Empire to win. An 'independant' Skyrim during the troubles caused by Jagar Tharn destroyed the combined forces of both Hammerfell and High Rock. They are much more dangerous even as a lone provicne than people give them credit for. By the end of the Stormcloak questline Skyrim in no longer asunder but united. They are also in a better position to ally with Hammerfell (which simply won't happen with the Empire, or at the very least is incredibly unlikely), and Ulfric is cunning. In my opinion he would still ally with the Empire to fight, he just doesn't want to be part of a vestigal Empire.
Let's look at real history now too: The Japanese armed forces were in absolute tatters near the end of WWII. And yet... the Allied forces were still fearing the prospect of a land invasion of Japan. Why? Fanatical population. That's what the Empire would have faced. The Thalmor did their homework driving out and then subsequently slaughtering all the dissidents (ask Legate Fasendil about that); all that's left are the fanatical or the fearful.
Reman never did conquer the Isles, the best he manged was Tribute. Tiber Septim needed the freakin'
Numidium to conquer them. The Empire had neither a Dragonborn nor a Giant Stompy Robot on their side this time.
Sure, the Nords may have been a force to be reckoned with... as far as matters of the Sword are concerned. But the Thalmor don't play that game. They go for the Dagger Behind The Back. And as shown, the Stormcloaks jumped exactly when the Thalmor told them to in order to weaken the Empire.
And all the rhetoric he expounds say otherwise to "allying with the Empire".