I just can't agree with your major premise. No heart or mind, not even a Jew's, is going to be won over with a contemptuous shout of "you stupid ass!"
Only Christ's grace can win over a jew or any other non-believer, but Bro Nat's unconventional style and no mincing of words directed at jews is sometimes necessary to knock the veil from their hearts, even if it's only momentary, so they can at least see to what he is testifying. Bro Nat "speaks the language" of the jew. He knows how they think and what is needed to make them lower their stubbornness and at least slightly consider what is being said to them.
What's so wrong about calling a jew a "stupid ass"? They are stupid asses. It's not vulgar, nor is it a sin to call them by such name. An "ass" is simply a stubbornly ignorant donkey. Our Lord called the jews "vipers".
Do you really think that St. Vincent Ferrer, who succeeded in converting more Jews than anyone since apostolic times, would applaud Nathanael's behavior—or your support for it? I don't.
St. Nicholas of Myra punched heretic Arius in the face at the Council of Nicaea. Pope St. Pius X punched a modernist heretic, too, did he not? Some behavior is necessary, depending on the circumstances and the person(s) to whom it's directed, in order to help set them straight or, at least, briefly snap them out of their detrimentally clouded mindset.