On original sin, if it is merely a deficiency (I’ve heard it described as an imcompleteness, too, in the Catholic view), then why would God judge people and send them to Hell for eternity? The Bible, taken altogether, answers that. The short answer, though, is our own sin, our own turning away from God in rebellion. There is so much mystery to much of this, but that we break God’s commandments, and our consciences bear witness to us doing it, that much at the least is clear. If we come to see sin as the Lord does, and our own sin, and believe that we’re under eternal condemnation for it, then we see we have a problem in that there’s no greater threat than Hell, and we need God’s forgiveness and salvation.
Nonetheless we bear its consequences because Adam and Eve somehow damaged their very human nature by their sin. And we are their inheritors for good or for ill. If they had remained sinless and undamaged, we would have acquired an undamaged human nature. Since they blew their very ability to give us a good inheritance, we have gotten from them many ills, both bodily and spiritual.
What about people who die before they have ever committed an actual, personal sin, e.g. babies? We don't know for certain, but we do know God is Good. In the Catholic funeral Mass for unbaptized babes, we trustingly commit them to the Mercy of God.