Assume that you can compare personal utilities and it turns out that this guy has below-average utility. [..] I suppose that the moral intuitions of most, though not all, people would be against killing him,
I understand why you say this, but I'm not quite sure I agree.
I mean, I certainly agree that most people, if asked that question in those terms, would say "of course not! killing this poor lonely friendless unemployed wretch would be wrong."
But I'm less sure that most people, if placed in a situation where they express their revealed preferences without framing them explicitly, would make decisions that were consistent with that answer.
And if I actually worked out what "below-average utility" means in terms that make intuitive sense to people... e.g., how much is this fellow actually suffering on a daily basis?... I'm genuinely unsure what most people would say, even if asked explicitly. Especially if our mechanism for comparing personal utilities, unlike the one I proposed above, does not arbitrarily conclude that each individual's lifetime maximum is equivalent for purposes of comparison, as I expect most people's intuitions in fact don't.
That said, I certainly agree with you that most of the people who are in favor of letting the hungry starve, etc., are not using any sort of aggregated utilitarian moral reasoning.
Consider the following facts:
This sounds a lot like the mere addition paradox, illustrated by the following diagram:
This is seems to lead directly to the repugnant conclusion - that there is a huge population of people who's lives are barely worth living, but that this outcome is better because of the large number of them (in practice this conclusion may have a little less bite than feared, at least for non-total utilitarians).
But that conclusion doesn't follow at all! Consider the following aggregation formula, where au is the average utility of the population and n is the total number of people in the population:
au(1-(1/2)n)
This obeys the two properties above, and yet does not lead to a repugnant conclusion. How so? Well, property 2 is immediate - since only the average utility appears, the reallocating utility in a more egalitarian way does not decrease the aggregation. For property 1, define f(n)=1-(1/2)n. This function f is strictly increasing, so if we add more members of the population, the product goes up - this allows us to diminish the average utility slightly (by decreasing the utility of the people we've added, say), and still end up with a higher aggregation.
How do we know that there is no repugnant conclusion? Well, f(n) is bounded above by 1. So let au and n be the average utility and size of a given population, and au' and n' those of a population better than this one. Hence au(f(n)) < au'(f(n')) < au'. So the average utility can never sink below au(f(n)): the average utility is bounded.
So some weaker versions of the mere addition argument do not imply the repugnant conclusion.