Hey, OP here. Anyways, for those who are saying I should take it as a joke--he was fully serious when he said it. He's not the best human being. He's done this before, and I don't know why he specifically attacks my son's kitten. He's threatened to let the cat outside so it can get hit by a car, has said he hopes it runs away and never comes back, and I even caught him trying to kick and/or chuck it out of a room. He repeated what he said to my husband, but claimed that my son couldn't take a joke and refused to apologize. My son was speechless when he said it and went to his room because he didn't want to be seen crying. He's only 8; he doesn't need to hear that his grandfather wants to kill his cat. My husband did the responsible thing and told my father that he thinks he shouldn't be involved with my son that much anymore.