Hume wouldn't be a Humean if rational self-interest were the standard. Stanford Encyclopedia of Philosophy writes:
These moral sentiments are emotions [...] caused by contemplating the person or action to be evaluated without regard to our self-interest, and from a common or general perspective [...]
Regarding Eugene's point about terminal value, I agree with the following clarification: the primary reason murder is wrong is because it deprives somebody of the rest of their life.
This still allows us to distinguish between murder and failure to create new lives, provided that we see a difference between someone who already exists and someone who merely might exist.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.