"Yeah, basically, but... what answer is there? Presuming we both agree that genocide is bad, we would subsequently agree that not-genocide is to be preferred to genocide. Regardless of whether the world is good or bad - as that is indeed a question I don't think anybody can really answer in its totality - I think if we can agree that not having something in it is better than having something in it, then we can agree a world without that something is better than the world with it"
You commit at least two fallacies here.
First of all, we can't assume that the bad could be taken out of the world and the rest left the same. For example, there's the free will argument.* Perhaps the very existence of free will is such a good that it outweighs the fact that people will misuse it and commit evil (and thus, there will be evil).
Almost everybody is familiar with these tradeoffs. For example, I could say, "Surely you must admit that a justice system where guilty people are virtually always convicted is better than one where they're not." And you'd probably agree -- but if I'm arguing that therefore, we should discard the fifth amendment and all the similar procedural guarantees, then I'm committing a fallacy, because it turns out we care even more about fairness and not convicting an innocent person or turning our society into a police state than we do about convicting every guilty person. So it's illegitimate to single out a single variable, and then trumpet that changing it would be an improvement, and ignore other factors.
Which is exactly what you've done.
So we CAN'T -- in the context of possible worlds that could have been created -- necessarily agree that a world without genocide would be better than a world with it, purely because we can't know what it would entail to create a world without genocide. I'd prefer THIS world without genocide, but I certainly don't know just how that would be achieved, and I can't pretend (and neither can you) to know what trade-offs would be involved in making that decision. It's mere empty rhetoric if you pretend you can.
You also committed another fallacy. You say we can't answer whether the world is good or bad. But that (in any case) is all God would be required to do -- make a world that is net good. If this world is good but there's one you'd consider better -- well, it's still good, so it would still be a good to make it. Why might he choose this one over that one? Who knows? Maybe because that one entailed no free will. Maybe because He loved the people in this one better and wanted them to exist. It's not up to us to know.
But let's be quite clear: if you feel sure that, net, this world is an evil, and making it and allowing people to be born to it is wrong, then we have it in our means now to end that ourselves (either through nuclear Armageddon or, less dramatically, forced universal sterilization), and are therefore just as culpable as He if we don't. If you don't think that would be justified, then perhaps the world IS good, withal, and allowing this is not an evil.
"If I misread then do correct me - but what bad rewards might these be? ...genocide, perhaps?"
No, the bad rewards I spoke of are precisely MY proclivity to sin.
Genocide -- if it is committed upon me -- is the sin of another, for which he is responsible. Yes, needless to say, the sins of others often have terrible consequences for people. This is part of why they're sins. That sin will have to be payed for. Which doesn't make me alive again, of course -- which is part of why the sin is so horrifyingly severe and important. Its consequences cannot be undone.
"And however many millions or billions of victims of genocide have existed since mankind arose would all consequently be suffering two things: unjust slaughter at the hands of evil actors, and punishment for something their ancestors did. Neither of these is the mark of a just world."
They're suffering for what the people committing the genocide did, not what their ancestors did. Yes, there is a causal link between the two, but the latter suffices. And yes, certainly it is not just for the people that they are being killed. But "just world" would have to be defined. This is not a world in which murder is impossible, but it is one where it will be finally and severely punished, so there is indeed justice in that sense. Whether it is a GOOD world -- well, see above.
"Do I, though [know God's law]? I can read the Bible, sure, but I don't necessarily know that the Bible conveys God's law."
Certainly you do. But my point wasn't that you know it's God's law (though on some level you do -- but that wasn't my point and would take us far afield). What I meant is that some part of the content of God's law is acknowledged by your mind, to yourself anyway, as right and wrong, and you still sometimes violate it. You do things to other people that you call wrong when they do them to you. My point was merely about culpability. There is enough to condemn somebody based just on what they knew was wrong and did, even without reference to what they should have known and did not.
*I have some technical problems with the free will argument, but they're unrelated and it's simpler than the analogous arguments I'd use, so I'm going with it for this example.