Of the three men who dominate the narrative, Tub is the protagonist. When there are three characters in a story, there is often a phenomenon of triangulation, where two characters ally themselves against the third. That seems to be the case when the story opens as Kenny and Frank treat Tub cruelly by nearly hitting him with the truck after making him wait outside in the cold for an hour. Their mistreatment of him continues during the hunt as they mock him for his weight.
After Tub shoots Kenny in what seems to be a sort of pre-emptive self-defense, the triangulation seems to shift to Frank and Tub against Kenny. They don't respond with alacrity to get help for him. The relationship among the three men is extremely dysfunctional; their inhumanity towards each other precludes any bonds they might share.
While the three main characters share the spotlight in "Hunters in the Snow," Tub is primarily our main character, and thus, our protagonist. The protagonist is the character with whom the reader most identifies and learns the most about his or her development.
We first meet Tub as he stands on the corner waiting for his friends to arrive. We see the most development of his character; for example, we know he has a weight problem of his own making which he blames on glands. We witness his abuse at the hands of his so-called friend Kenny. The reader feels sorry for Tub and possibly identifies with him. Later, we watch him grow from allowing himself to be a pushover to taking charge of his abusive friend Kenny. We see his mindset change as he develops a bond with Frank. Finally, the reader tends to forgive his ambivalent attitude towards Kenny's impending death. The ironic tragic situation is not Tub's fault.