Machete, my 14 year old Dachshund, has recently started biting out of aggression and attacking Elvira, my new Sussex Spaniel puppy. She has already drawn blood from her once and me twice. She wouldn't ever think about going after Agrarian, but I'm really worried. I've been considering euthanasia. Would that be cruel? She is old and has just become so mean! I don't know what else to do. It is out of the blue and very odd!