"What matters to an active man is to do the right thing; whether the right thing comes to pass should not bother him." -- Maxims and Reflections.
This is what Professor Hanson and his ilk ignore. We can never be certain that our actions will produce the result we want; to enter into historical counter-factuals is to wade into an intractable bog. All we can do is act according to the principles of justice: we know it is wrong to slaughter a quarter-of-a-million innocent people, even if we might fear the consequences of our not doing so.