The serpent said to Eve, "You will be like God, knowing good an evil." He was offering her the power of independent moral judgment. Previously the boundaries of her conduct were set by God. God's commandment was that she and Adam not interfere with that. They were not to eat of the tree of knowing good and evil.
The narrative permits several readings, some literal, some metaphorical and some that blend the two. However you read it, the central point is the same. Mankind's problems stem from taking upon ourselves the definitions of good and evil, taking the matter out of God's hands.
The trouble is we continually get it wrong. Biased by self interest, blinded by ignorance or simply because we're perverse, we call bad things good and good things bad.
The same old lie just keeps rolling along, causing one disaster after another. In Jesus' day, Pilate offered the cynic's defense: "What is truth?" He pardoned Barabbas, condemned Jesus and washed his hands of the matter. Jesus said:
“You say that I am a king. In fact, the reason I was born and came into the world is to testify to the truth. Everyone on the side of truth listens to me.”Pilate made a colossal error in judgment. In the process he invoked the then-fashionable idea (and fashionable today) that truth is an abstraction and people have varying ideas about what it is, one about as good as another. Today people say things like "true for you but not for me--that's only your truth."
“What is truth?” retorted Pilate. (John ch. 18)
Of course the idea has always been morally bankrupt. It is the same line, in different words, that the snake sold to Eve in the garden. It's up to you. What's true or false, good or bad, right or wrong is within your purview to decide.
When there is no transcendent standard, people never agree on what is right and wrong. How could they? Their own biased judgments in the matter can scarcely agree, since people are biased in different ways. The homosexual has no use for ancient warnings against what he likes to do, and perhaps the person who points out those warnings does so with less than godly charity at times--being biased himself, against gays.
So it's all a complete and utter mess. The idea that God's standards are for our good not his and they help us if we follow them is generally discarded in favor of doing things our own ways. The same is true of any other firm standard that may be proposed, whether secular or religious. People decide for themselves which bits they like and which they don't.
That there really is a transcendent moral standard that applies to all people equally can be shown pretty easily. Punch a moral relativist. When he says "Hey, you should not do that!' you simply reply, "Who are you to impose your moral standards on me?"
Harm to one another, and ourselves, is what God is mainly interested in avoiding, when he sets forth rules for our conduct. Of course it annoys everyone to be told that something is for our own good, and people readily rebel against it and adopt their own private moral judgments instead. But in this case the thing really is for our own good, which is why the devil has opposed the idea from the beginning.