Pacelle, writing in his blog, A Humane Nation, said of animal-abuse industries: "Today's apologists for cruelty are most sophisticated and deceptive, now laying claim to the argument that they are the best defenders of animals, and that when it comes to caring for them, they know best."
He said the evidence can be seen when trophy hunters claim to be helping conservation, or factory farmers claim that best science supports their position and that animals in warehouses are protected from predators and disease threats, even though the animals cannot even turn around or engage in normal behaviours.
"But giving them a run for their money, in terms of self-centered rationalisations, is the horse slaughter crowd ...
"If you know the details of the horse slaughter industry, there's an inescapable conclusion that it's a disreputable, predatory industry, gathering up horses from all sorts of sources and turning them into meat exports for profit.
"These people have not a thing to do with responsible animal ownership or proper care. They actually get in the way of responsible care and re-homing of horses when they outbid horse rescue groups at auctions, because there's a profit to be made in exporting the animals for slaughter.
"They see horses as commodities on the hoof, and they take nothing more than a utilitarian attitude toward our fellow creatures. Thank God there's no active slaughter industry for dogs and cats, since we'd all have to watch our pets around these people all the more."
Pacelle cited the case of a loved Arabian horse, Rhapsody Rhose, reported this week in the Topeka Capital-Journal in a piece looking at horse slaughter.
The horse was owned by Jaime Cowan, but Rhapsody was reportedly sold by a temporary caretaker without Jaime's permission.
She soon ended up in a Mexican slaughter plant.
"It wasn't a hardship case, or an unpleasant necessity," Pacelle said.
"It was just deception for someone's personal profit, and an innocent life was chewed up in the process.
"So when you hear high-minded arguments from the horse slaughter crowd, remind them of Rhapsody Rhose and thousands of perfectly healthy horses who deserved so much more than greedy, opportunistic, unethical, ruthless people."