They transferred their allegiance from God to Satan, from this government of God to this sub-government of death and darkness ruled by Satan. There was no way to scramble back. God's Word had to be fulfilled. God was bound to put them to death for their sin. He allowed the death processes to set in upon man, the breakdown within the cell structure known as the aging process.
Man now lives under the government of death and darkness.
He's alienated from God and finds himself with no way back. Hope in the Darkness God could have obliterated His entire experiment when Adam disobeyed, but He didn't give up on man. God would show His supreme love to man and bring him forgiveness. God would take upon Himself a body of flesh, live among men, take the guilt and responsibility of man's disobedience upon Himself, and die for it. Man's sin would be punished.