So would it be fair to claim that we are responsible for the deaths of people starving in the developing world whom we could quite easily provide food and water for with no great cost to ourselves?

If you know of a way for me to do that, I'd like to hear it. I think everyone would. We can, and I have, contributed to charities, which unfortunately do not solve the problem, but we have little influence over the system that creates malnutrition.

If you really insist that Greg possesses the supernatural ability to predict specific outcomes, then the question becomes one not of Greg's choice, but of Greg's belief that he possesses divine powers.

That's fine, we can look at it that way too. If Greg believes that he knows the exact outcome of either choice, does he also believe that he can assess the value of all the lives in the situation? Does he believe he knows that there is no Jonas Salk or Mahatma Gandhi among the five he believes he will kill in exchange for the 40 who he believes he knows contains no Caligulas or Hitlers? Does he believe he can value human life by counting?

I don't think he can know those things any more that he can know specific outcomes, but if he believes he can, he's doing what most of Greek tragedy concerns itself with: having hubris. His belief in his omniscience will cause tragedy.