To me it has always tainted the duel. Now I'm not Ulfic's biggest fan but he neither a coward of a fool so it was a deliberate gesture. This gesture has made his victory iIMO less honorable. I know there is disagreement on this but that's thepoint. Why give those who doubt legitimate fuel inmany eyes?

My theory is he felt demonstrating the power of the voice would give him more awe, respect and legitmacy so it was worth the risk of being thought dishonorable by some.

I think it was a move to terrify the other Jarls, and let them know that he was prepared to challenge (and kill) anyone who is elected High-King at the Moot but him.

It was a Nord duel, I would assume that by Nordic traditions a duel to the death would allow them to use any weapon to their advantage.

Remember that Nords are based on the Norse in many ways and their definition of honor is different than a samurai's definition of honor.