I think that Lawson follows some of his basic idealism and desire to affirm solidarity in how he describes the behavior of the dogs. The use of selective language as a technique to accomplish this is evident. Tommy represents the force of good and of community. Tommy is loyal to the men, never doubting their sincerity. When they flee from him, he pursues thinking a game has commenced. Tommy is initially described in lovable terms, reflecting a purity and sense of innate goodness that cannot be taken away from him:
rather an overgrown pup, a big, foolish, four-footed mate, who was always slobbering round them and lashing their legs with his heavy tail that swung round like a stock-whip. Most of his head was usually a red, idiotic, slobbering grin of appreciation of his own silliness. He seemed to take life, the world, his two-legged mates, and his own instinct as a huge joke.
Tommy being "an overgown pup" and described with an air of lightness such as as "his own silliness" as well as "his own instinct as a huge joke" reflects a description in which Tommy cannot help but be loved. In the end of the story, Tommy is described as "the great, idiotic mongrel retriever," reflecting how Tommy cannot help but be loved. There is little malevolence in Tommy and his description throughout the story matches this.
Certainly, this cannot be said for Tommy's "other." The antagonist to the story is described in terms that reflects a sense of cruelty and preordained evil. Consider the language Lawson employs in describing the initial confrontation between both dogs:
...there was a vicious yellow mongrel cattle-dog sulking and nursing his nastiness under there--a sneaking, fighting, thieving canine, whom neighbours had tried for years to shoot or poison. Tommy saw his danger--he'd had experience from this dog--and started out and across the yard, still sticking to the cartridge. Half-way across the yard the yellow dog caught him and nipped him.
"Vicious," "mongrel," "sulking" and "nastiness" are the terms that Lawson uses initially to describe the dog that represents malevolence in the story. Lawson is determined to use language that reflects a force that does not believe in solidarity or a sense of idealism. Lawson's technique of selecting language to help accomplish this dichotomy is how he is able to bring about a sense of moral structure to the story. In this light, one can see the story as a type of morality play whereby Lawson through language is able to reward the righteousness and punish those that seek to undermine such goodness in the world.