Humans are the only species that are motivated by vengeance to do damage, and what we humans do is sometimes inhumane.
It is not true they we are the only species that are motivated by vengeance. I have seen this in my dogs and it is certainly true of most primates. If these were a wild species I would agree with you. But these are feral domesticated farm animals. Likely these guys are hired by a park or agricultural administration. Feral hogs are bad for both farms and wildlife.