Simply put, Slim does nothing to stop the shooting of Candy's dog. Slim is a "doer." He is not one who resides in the vision of what can be. He is of what is. He is of what is in front of him, and in this, Slim concludes that the dog is really of no good to anyone. To a great extent, Slim's voice is what enables the dog to die. His absence of voice is akin to a court to which Candy appeals for a "stay of execution" for his dog, but to no avail. Steinbeck's inclusion of this might reflect how there is daily cruelty in the world, even within good people. Certainly, Slim is a good man, someone who proves to be loyal to George throughout the novella and especially so at the end. Yet, one of the novel's central idea is how to assess good people who do bad things. George and Lennie are but two examples of this. Slim's silence when he could have stopped the dog's death might be another example of this. There is a state of being in the world that Steinbeck seems to be suggesting whereby human beings are complex, capable of the most horrific of realities as well as the most redemptive of actions. In this, Slim's character is expanded for while he might be a heroic character, he is one who does a bad thing in allowing for Carlson to shoot Candy's dog.
slim never things about shooting Candys dog its Carlson that wants and does shoot him!!!!!!