When a soldier shoots another soldier, that is murder even when it is allowed. The fact that it is allowed is immaterial to the definition of what it is.
For example, there are laws against murder. But if, while defending yourself, the other is killed that is not considered wrong, in most cases.
You are right that taking life is taking life. And I would agree that a case could be made for murder in some cases with soldiers.
So the real question is is it killing a fetus or murdering a fetus?
No one debates there is life.
People do in fact debate whether or not there is human life.
If you choose to define it as murder, that's up to you, knock yourself out, but the state has power and authority - you don't.