aes1004- I had similar comments from some older people that I knew from church who were my friends on FB. I responded that it was very rude for them to insinuate that my pets should be killed and asked what they would do if someone posted that on a picture of a pet dog. They apologized and then never commented on any of my posts again.
I think people just say crap like that and don't actually think about what they are saying or that they are saying it about someone's pets.