@YJ,
"Personally, I think it's a stupid question and I refuse to engage. Acknowledging it's humanity from the onset is the only solution that isn't entirely open to arbitrariness."
The problem is, with your definition, and a support of abortion's legality, your choice of which murder is *wrong* (or should be illegal) becomes entirely arbitrary.
If you're going to support abortion and not (what we all agree is) murder, you have to be arbitrary somewhere.
"Now, if you want to use a more commonplace definition of murder (phrases like "against the law" and "malice of forethought" come to mind)"
First, quick note -- it's "malice aforethought." ; ) Not that you should know that.
Second, while I accept that several popular dictionaries define murder with respect to the law, I think there are serious problems with that definition, in terms of how people actually use and think about the word. Not that it's always wrong, but I think there's an important other definition, (logically) preceding any human law, which would be the wrongful killing of another human. This removes the tautological aspect from discussions of whether a particular type of killing is murder. Anyway though, this is just a note about how the word is used; I'll agree that arguments about definitions in the context of broader arguments are often unhelpful.