In the description of the creation of the world as recorded in Genesis, God said several times that it was "good."
Genesis, chapter 1:
...."Then God said, "Let there be light''; and there was light. And God saw the light, that it was good..."
...."And God called the dry land Earth, and the gathering together of the waters He called Seas. And God saw that it was good."
But Obviously, Something Happened
If you've lived long enough in this world, you've noticed that it is no longer "good."
Why?
When God created mankind and placed them on the earth, they lived in a state of perfection and walked with, and enjoyed God freely. He made only one demand on them, warning that if they disobeyed him, the result would be death.
The Lie
Satan, a fallen angel filled with hatred for God and for all that God cares about, came to them and suggested that what God had said to them was not true:
And he said to the woman, “Has God indeed said, ‘You shall not eat of every tree of the garden’?”
Genesis 3:1
It is extremely important to remember this part of the story because this is always the way Satan lures mankind into sin: he tells them what God said is not true.
"Then the serpent said to the woman, 'You will NOT surely die. For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil.'"
Genesis 3:4-5
They fell for that and believed Satan's suggestion that it would actually be to their advantage to disobey God. And so the first sin took place, the sin of UNBELIEF. They took the fruit.
It wasn't an apple. Satan has so distorted the story so as to make it sound ridiculous. No, the particular fruit had a property which opened them up to the knowledge of good versus evil. God had wanted them to remain in an innocent state, but now that was no longer possible.
The Horrific Results Of Sin
This act of rebellion brought the curse of death not only upon the two, but the death principle affected them completely, even to their very genetic makeup. And that meant that it was to be upon everyone who would ever be born of them. It becomes obvious that it was a genetic curse because subsequent generations would show plainly that the original perfect genome has mutated with each subsequent generation. In the very beginning, there was no law forbidding intermarriage within families because the genome was still fairly healthy and it would take several generations before genetic diseases would become apparent. Later, when God gave the law, he mandated an end to marriage with family members. They couldn't have known about genetics then, but it makes total sense to anyone reading about it now.
But death didn't only come upon them. Sin brought death into the whole world. The death principle took over everything. So now, even animals, even plants came under its curse. No living thing was exempt from death. Death took over the entire world. God's creation deteriorated to the point where sickness and death, violence and death, accidents and death, and all manner of death became the accustomed way of life on planet earth.
More Effects From The Curse
The worse effect, though, was that sin now stands between God and man, and mankind, being born IN sin, has lost fellowship with God. And upon death, man cannot enter God's heaven because man is corrupted. And corrupt beings go to hell, Satan's domain.
God Himself Has A Problem
Satan thought he won in his effort to destroy God's creation, and surely he had succeeded in causing the separation between a holy God and a corrupt mankind. So now GOD had a problem. Every human being's destiny was no longer to walk with God, but to die and be taken by Satan to hell, a place that experiences no fellowship with God whatsoever. This was, it HAD to be, the fate of sinful mankind who had lost righteousness, lost sinlessness, lost holiness, and therefore was by nature unfit for heaven and cut off totally from God.
So WE had a problem without a solution.
And God had a problem because his creation was lost. Every descendant of Adam was sin-sick, under the curse of death, and separated permanently, in this world and in the one to come, from God.