Considering the tar was applied hot, I don't think you can say it was a "non-violent" punishment. My guess second degree burns would be certain and third possible over large portions of the punishee's body.
My understanding is that it was a quite painful version of vigilante Justice.
I wouldn't think 3rd degree. I have never heard of such an event leading to death.
To be clear, I am saying that sometimes, if law adn order are absent from society, vigilante justice is called for and better than letting violent criminals terrorize society.
Assholes like Newsome.