A friend once commented that God was unfair to Adam because He did not fully explain to Adam the consequences of disobeying His command not to eat the fruit of the Tree of Knowledge. My friend's point was that if Adam knew what terrible consequences would follow his act of disobedience, he would not have done it. Adam was at a disadvantage because he did not have full knowledge.
I pondered over this and came to the conclusion that knowledge of consequences and fear of consequences should not be the basis of good or bad behaviour. Most animals behave in an "approved" way because they fear the consequences. Man should obey his God because He is God. Man should obey God out of love for his Creator and not because his Creator wields a big stick.
I wondered if Adam's sin was a grave sin and came to the conclusion that it was. Adam was enticed by the idea of being as knowledgeable as God. He wanted to be a god and so his was a sin of pride. So just as Lucifer fell, do did Adam. But Adam was the lucky one because he was given redemption and entry into Heaven through the Sacrifice of the Son.