After Adam HaRishon ate from the forbidden fruit, he was now susceptable to death. Before this, he had the potential to live forever but now his lifetime would span a certain number of years.
The Ramchal in Derech HaShem explains what this means. He says that before the sin, the soul was so powerful that it could fully purify the physical body if used properly. But after the sin, the soul became less spiritually potent and we became more in touch with our bodies than our souls, and so the souls could no longer uplift the body fully.
Therefore, HaShem had a realm created whereby body and soul were separated so that they could go through a ‘wash’ and be purified. This realm is what happens after death; when body and soul separate.
Therefore, it was not only that death was created because of the sin, but the need for death was created.
And what we can take out of this is the depth of Torah!
