The story is deliberately written so that none of the characters come across as sympathetic. Each is selfish and self-absorbed, looking to the others for validation and yet doing nothing to deserve that validation. Even the old farmer is unsympathetic, not seeming to care that Kenny has been shot. Tub should be the sympathetic character, since he is the protagonist and picked-on by the others, but instead he proves to relish in his faults and actually wants to be insulted. Frank is going to destroy his own loving marriage because of an affair with a young girl, and Kenny simply abuses Tub for fun, not caring about the consequences. In the end, the only sympathetic figure is not a human at all:
...a large black hound with a grizzled snout ran out and barked at them. Every time he barked he slid backwards a bit, like a cannon recoiling. Kenny got down on all fours and snarled and barked back at him, and the dog slunk away into the barn...
The dog is the only innocent figure in the story. It has no malice and no motive to hurt anyone, and it is killed out of pity (the farmer) and to create a new, cruel joke on Tub (Kenny). However, that joke has terrible consequences.
Tub takes the place of Kenny and Kenny takes the place of Tub at the end of the story. They exchange attributes after the climax.
I think at first it's Tub but towards the end you feel sympathetic towards Kenny.