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.