Um, I'm sure you are aware that many words have different meanings in different contexts... and you are CLEARLY using the term "murder" in the legalistic sense.
That is, in the definition 1 : to kill (a human being) unlawfully and with premeditated malice
And abortion isn't unlawful, therefore, it isn't murder.
Quite right, the law DIDN'T used to recognise slaves... but, from a legal context, we can only operate within what the law defines TODAY... and TODAY it doesn't recognise abortion as murder. If the law chnges tommorrow, and says that abortion IS murder, well then there will be absolutely nothing wrong with your initial statement. However, its no good saying "well the law SHOULD change, so I'll pretend as though it has"