You're confusing psychopathy with simple callous cruelty spawned from the objectification and commodification (in terms of money) of non-human persons for human gain. Not every slaughterhouse worker is a psychopath. They simply have a different standard for what constitutes a member of the moral community, just as slaves weren't considered human until the southern states needed more representation, at which time, each slave became "3/5" of a human being.