As any observers of wolves know, wolves spend little of their time killing. Most chases of prey end in failure. The chance of success increases when more than one wolf hunts, so giving benefit of the doubt to the metaphor, one killing machine is often not enough. No one would really argue that. In fact, the only machines we know about were created by humans.
Humans can also be part of a processes that can be described in mechanical terms.
If a killing machine metaphor is appropriate, it could be argued it is the rancher and other humans involved in the process are cogs in a killing machine, not the wolf. Wolves would factor in as a minor outside disturbance to the machine-like process of livestock production, killing, and human consumption. Many ranchers tend to forget that humans consume far more flesh than all the wolves, bears, and nondomestic cats in the world. They tend to forget that the fate of their cattle is the slaughterhouse followed by consumption more thorough and efficient than that of any wolf. Throughout history, some human beings have been devalued and denied moral status by use of metaphor.