Whether killing is murder requires an ethical judgment to be made. If a criminal on death row is executed, we would not say they were murdered. At least, not until it is shown that they were innocent after the fact; in which case those who falsely accused him, botched evidence, mishandled the case may be ethically considered to have murdered him, or at the very least have committed negligent manslaughter, though legally speaking there are limitations.
Also, murder applies only to beings considered persons. As animals are not considered persons by most, it is a category error to suppose that they can be murdered.