Original sin is why mankind is in a fallen state. However, that's not our fault. We're born into this system but we're not going to be held accountable for it. We have our own sin which is why we're under the judgment of God.
Answer 2
He Became Sin for Us
Christ had to become sin for us. This has a meaning which goes beyond the symbolism of Christ on the cross. It goes back to the nature of the matter of which the universe is made.