Emotional attachment is a hard thing to deal with. When those attachments are formed, regardless of whether it is another person, a pet, or good 'ol Bessie there chewing her cud for the third time, it is painful and difficult for those attachments to be severed, especially by death. Remember, cows weren't domesticated because of their being warm, cuddly, and amazing at catching frisbees. They were domesticated because they were a good source of meat. It was a pragmatic development on the part of Thog or whatever that ancient person was named to make getting food for his family that much easier.
Humans have a tendency to deal out hurt rather than accept it.
I find this understandable, though not laudable. But until the day when we all become vegetarian and hell freezes over, someone will have to play butcher. I don't envy them, and I shan't blame them for acting in a way to protect their sanity. Call me selfish, but I do place humans above Bessie or Spike or any other pet, animal, or plant out there. I have to.