From what I understand, being unable to empathize with animals at all is actually a potential symptom of sociopathy - after all, if you have no empathy for other human beings (apologies if I'm oversimplifying it) then why would you have an ounce for "lower animals"?
This is not the same as being desensitized to the slaughter of animals, such as with hunting and farming. Though there's a point where that can go too far, too.
Then, of course, there's the psychopaths who, generally speaking, start off by torturing animals for amusement.
Of course, I could be talking out of my ass about all this, and if I am, I apologize.