What is the title and author of the short story about a man who poisons his flock wife and himself?

The story starts with the man poisoning some meat to get rid of a predator that has been preying on his flock of sheep <-- pretty sure he has sheep. He ends up becoming infatuated with watching animals twitch and die from this poison and at the end he gives his wife a bowl of ice cream with poison in it just so he can watch her die as well then brings out a mirror and also eats poison and watches himself die. I need it for a literature project.