I agree with you that murder statutes are state issues, however in this case alone we're dealing with a 14th amendment issue if we define the unborn baby as a person. It would outlaw the practice in the whole nation. IMO it is a very limited, distinguishable set of circumstances that justifies federal action.
I'm sure there would be differnt "degrees" of abortion, just as there are different "degrees" of other murders. And some states would still try to facilitate the killing as much as possible. But if the SC acknowledges the personhood of the baby before birth, that would open the door for the Constitutional issue of "equal justice under law."
In other words, more decades of struggle. Still, it would be a tremendous thing. God grant it!