And does being "stronger" necessarily equal being the better ruler? Sometimes being diplomatic is better than being a military general.
Ulfric killed Torygg because he KNEW he could, not because of some moral high ground.
In the Nordic cultures, Yes. Being stronger is an equivalent in being a better ruler. Nords are fierce, strong and an enthusiastic warriors and many become warriors, soldiers and mercenaries. In their culture they abide by a sacred tradition, a Nordic tradition, that allows someone to challenge their king to a duel in legal combat to the death if they deem to be unworthy to rule, or show weaknesses. The winner becomes the new King once he gets the full support from the other Jarls and the Moot declares it.
Torygg was not diplomatic whatsoever. He was a mere a puppet king of the Empire addicted to Imperial coin, and being a military general helps a lot since you are technically the boss. It gives you far more experience in ruling a country. Kings don't just sit around and look pretty with their expensive jewelry and clothing that's paid for by the hard working citizens of the country. He is suppose to lead his armies in case if theirs war.
You know I would have no problem supporting the Imperials if I knew the leadership will go into the right set of hands. If Brina Merilis was to become queen of Skyrim under Imperial victory, I would support this... but handing over Skyrim to Elisif? The woman who is becoming far too friendly with the Thalmor by enjoying their luxury parties? No.
Like what Ulfric said: "We should risk letting those milkdrinkers put Thorryg's woman on the throne? She'll hand Skyrim over to the elves on a silver plate."
Ulfric is right no matter how pro Imperial you are... Elisif is a pawn to the Thalmor. Shes not only the puppet of the Empire, like her husband was, but shes being a puppet of the Thalmor! NOT GOOD FOR SKYRIM!