One interpretation of original sin and the Fall
In the first kind, God lets you know good and evil for yourself, in the second kind, God gives you His perspective on good and evil, ensuring your perfect goodness (Gen 3:1-5).
So Adam and Eve had a kind of free will where they were guaranteed to be perfect, so long as they didn't choose to work out good and evil for themselves. But God left them the freedom to work out good and evil from their own perspective, and they chose to 'opt out' of His protection. That destroyed their relationship with God, because without looking at things from God's perspective it is impossible not to sin if you have the ability to think in terms of selfish interest (Rom 7:18-24).
But why not just redo the whole thing with a different Adam and Eve? And how come we don't get to be in the Garden and decide this for ourselves (isn't that unfair)?
I think that without experiencing sin and evil, eventually *everyone* chooses to 'opt out' of God's protection, because we just don't know well enough why we should let God protect our choices. So we all share in Adam's sin indirectly, in the sense that we are all born with personalities that would have Fallen if we had started off in the Garden (but after our life, we have learned not to want or do that).
Christ's atonement lets us go back to a state where we see people in the way that God does, which puts perfection within the reach of anybody without regard to their former works, achievements, personality and character traits.
Check out Romans 11:32: "For God has bound all men over to disobedience so that he may have mercy on them all". Maybe we could have started off in the Garden like Adam and Eve, but God skipped this so He could set history up in a way that could save the elect (in my interpretation, anyone who God knew would not resist what Christ had done for them on the cross).
Link to some more speculation on original sin and the fall...