(Some straight-up philosophy of religion for your weekend.)
When philosophers talk about the Problem of Evil (aka "the POE"), they sometimes cast the question like this: "Why did God create a universe in which Adam chose to sin rather than a different universe in which Adam did not choose to sin? Was there no possible universe God could have created in which Adam did not choose to sin?" Then they go on to discuss these questions.
I think this is a confusing way for philosophers to cast the issue.
The reason that I think it is confusing is that it implies that God, in an act of creation, makes an entire world-history, an entire possible world with all that happens in it. I call this the Fallacy of the Clickable Universe. The picture it always gives me is of a pretty Microsoft Desktop arrangement, with a tasteful blue background, and all the possible worlds laid out on it as little icons. God has a mouse. He decides which one to create by clicking on it, and when he does so, that whole universe, history and all, is then fated to come into existence.
But that's not right. If we take human freedom seriously at all, we have to believe that God doesn't make the whole history of a world, when that world involves causally efficacious free beings, by his own omnipotent act of creation. The history of the world is made at least in part by the choices of those free beings. So God doesn't make a world in which Adam does X or doesn't do X. God makes Adam. Then Adam decides whether or not to do X. The reason God couldn't just choose to create a universe in which Adam didn't sin is not because there is some heavy fact about possible worlds according to which Adam sins in all possible worlds. The reason is much simpler than that: If God is going to create a free being at all, God doesn't decide whether the being sins or not. The being decides.
I think one source of this philosophical inclination is the notion that there must have been something about Adam that specially "accounted for" the fact that man sinned. Then the reasoning is pretty simple from there. Why didn't God instead make some different being, call him Adam*, who didn't have this fatal flaw? Then we would have a universe in which Adam didn't sin. But that, too, is the wrong way to look at it. Adam didn't sin because of something in himself, some dark bit of his nature that God, for reasons mysterious, chose to insert, that pushed him to sin. That, again, would be to take the full freedom and perfection of unfallen man with less than full seriousness. Unfallen man didn't even have a sin nature to deal with, like we do. True, Satan helped matters along with the woman, and then she got her husband into trouble. And poets and theologians from Augustine to Milton have rung the changes on what that all might have looked like and been like. But in the final analysis, the real source of man's sin was man's choice, an originary cause, not a determined billiard ball getting bumped along as part of some other stream of causes. So, no, God couldn't have created some different being who would have been guaranteed not to sin. He made a perfect, free, being, knowing that he would sin, and the story goes on from there.
Fortunately for all of us, of course, God didn't leave it there. He did something about it; he paid the final price. And so in the end, he gently takes up the pen and continues to write the story. When he lets us write parts of it, we blot the page often as not, which is a hard thing to have to admit. But apparently, as C.S. Lewis says, he thinks the game worth the candle. Hard as that is to believe, even we fallen men have the fearful burden of freedom to take up and go on with. May we bear it well.