How can we be expected know righteousness from unrighteousness if we have not actually experienced it? This may be an age old question which can be shown to be at the root of all evil.
Since man was made in the image of God, he has a sense of loss and conscience regarding what right and wrong is. Depending on his individual circumstances of life, he may also have a corrupt heart and conscience which will prevent him from seeing or appreciating any real concept of love, goodness or forgiveness.
It was inevitable that man would “fall”, more than this, it was the thing anticipated, if not intended, and even further, perhaps the intended design.
God could not create evil, but He could create a creature who was quite capable of it, given any set of circumstances which arose outside of God’s nature. One simple wrong choice, given that he HAD a choice (and Adam did not, his “choice” was inevitably going to be the wrong one), and the “game” was irrecoverably lost.
Once all was lost, man had then to “live” in this environment which denied the very nature of God, denied by thought deed and word, producing the works of the flesh. Man was subjected to futility from the beginning, to cause Him to reach out and seek God, that by his experience of good and evil in this world, make the faith choice for good, and progress into a new environment whereby his faith would be tested.
Man has a certain knowledge of right and wrong, and God will hold him responsible for that; but putting that aside, he never really had the choice for good, he never really ever had a “free will”, because he was a slave and servant of sin, of his own sin nature. But it is the acceptance of the truth of this, in conjunction with the truth of the way out of the predicament, which makes him then acceptable to God.