I think that Carlson's importance to the story is to represent that force of destruction that is intrinsic to the modern social order. Steinbeck shows Carlson to be a force that has no problem participating in the destructive element. Killing Candy's dog or engaging in the hunt for Lennie, Carlson has no problem with violence and a sense of cruelty. While other characters find this to be a challenge, something that requires deliberation, and even a sense of hesitation, Carlson has no problem with destruction and the unintentional cruelty that is a part of his identity. He is not someone who seeks to cause pain, but he is part of the mass of individuals in society that has little problem with it once it has been sanctioned. In a setting where much in way of painstaking attention is paid to individuals who try to nurture a sense of the creative in a world that lacks it, Carlson is the embodiment of the destructive element that is such a dominant part of the modern society. The fact that he is most visible at the two points where murder and the taking of a life is evident only goes to enhance this.