The problem is that "murder" is itself a legal definition. It is a fact that it is possible to kill another human being and not commit murder, and courts and juries decide that based on nothing but legal definitions.
Law in the United States, and in most of the civilized world, says that abortion is not murder, and therefore, it is not. Change the laws, and it might well become murder, and then you might find immense public pressure to change the law again. After all, it's in that context (public pressure through demonstrations and so forth, along with many, many legal proceedings) that eventually culminated in Roe v. Wade.
Yes, this could change again. That's the nature of human institutions. What I am trying to say is "abortion is murder" only if we humans say it is, and it isn't, only if we humans say it isn't. God is silent on the issue.