While Tub is the protagonist, one can assume that Kenny is the antagonist.  An antagonist in a story is the character who opposes the protagonist.   While Frank, the third character in this story,  does participate in teasing Tub about his weight, Kenny is the instigator of the verbal abuse and physical torment.  Frank is much more of a follower of Kenny's lead as they leave Tub behind in the snow and call him names.  In fact, later in their evening, Frank and Tub seem to bond.

Later, the confrontation between Tub and Kenny becomes violent.  Kenny does not explain his reasons for shooting a dog, and, in an attempt to be intimidating, he points his gun at Tub.  Tub reacts in fear and shoots Kenny.  The audience does not find out until later that the farmer had asked Kenny to shoot the dog.  At this point, Kenny is bleeding to death while Tub and Frank half-heartedly attempt to find a hospital.  While the reader feels for Kenny, his earlier behavior makes it easy to see him as the antagonist.