I do believe there is merit in my answer which fits both God's character and the witness of scripture, but it always seemed like there is more to be understood. Thai is answer can be summed up this way: to have knowledge of good and evil is to be closely acquainted with good and evil.
We are familiar with the description of marital intimacy in scripture ("And Adam knew Eve his wife; and she conceived" etc.). We can apply this to the tree and see the result of disobedience was an immediate intimacy with evil. But it would also have to be an intimacy with good.
So how do we wrap our brains around all this? The answer may be found in both the understanding of spiritual beings and in the Lord's own temptation in the desert.
First, let's step back and look at Satan's lie in the Garden:
For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods [Elohim, the name of the creator God], knowing good and evil.
The promise was an exalted state making them equal with God. This was Satan's desire.
How you are fallen from heaven,
O Day Star, son of Dawn!
How you are cut down to the ground,
you who laid the nations low!
You said in your heart,
‘I will ascend to heaven;
above the stars of God
I will set my throne on high;
I will sit on the mount of assembly
in the far north;
I will ascend above the heights of the clouds,
I will make myself like the Most High.’
But you are brought down to Sheol,
to the depths of the Pit.
Satan's eventual end is eternal death. This would have been Adam's and Eve's fate (and ours) if not for the death, lack of decay, and glorious resurrection of Christ, the seed of the woman!
Their sin was disobedience and a desire for equality with God. Let's like at one who had equality with God and how he viewed his position:
Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men. And being found in human form he humbled himself and became obedient unto death, even death on a cross. Therefore God has highly exalted him and bestowed on him the name which is above every name, that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.
We now turn to the Lord's temptation in the desert (Matthew 4). The first temptation came in the form of food as it did with Eve (“If you are the Son of God, command these stones to become loaves of bread.”). Now we turn to the final temptation.
Again, the devil took him to a very high mountain, and showed him all the kingdoms of the world and the glory of them; and he said to him, “All these I will give you, if you will fall down and worship me.”
They we Satan's to give. But what did we just read in Philippians 2? Every knee will one day bow to Christ. The kingdom's Satan offered will eventually be Christ's. In between was the cross.
Then the seventh angel sounded: And there were loud voices in heaven, saying, “The kingdoms of this world have become the kingdoms of our Lord and of His Christ, and He shall reign forever and ever!”
Can we apply this principle to the knowledge of good and evil?
Joshua the son of Nun, who stands before you, he shall go in there. Encourage him, for he shall cause Israel to inherit it. ‘Moreover your little ones and your children, who you say will be victims, who today have no knowledge of good and evil, they shall go in there; to them I will give it, and they shall possess it.
Taking this principle back to the Garden, the Knowledge of Good and Evil, Adam and Eve were innocent. They were babes in regard to life and the things of the Lord. They could not fully comprehend the implications of rebellion and sin. They did not know of good and evil.
The prohibition against eating the fruit from that tree did not mean they would never have access to that tree, it just meant they were not to have it at that time. We have discussions which we deem "for mature audiences only," even in the Body. If we talk about marital intimacy, we do not do so in front of young children. They are not able to rightly discern good from evil.
This applies to many natural things. Why shouldn't a hungry child just take his sister's cookie? He doesn't understand. When he is old enough to understand that stealing is wrong, and willfully steals, he is acting recklessly. If he would only wait, he would get his own cookie.
The mature in faith are able to discern good and evil. Doctrines come in two forms: milk and solid food. The deeper doctrines of the faith and higher understanding can be dangerous to the immature and weak believer.
For though by this time you ought to be teachers, you need some one to teach you again the first principles of God’s word. You need milk, not solid food; for every one who lives on milk is unskilled in the word of righteousness, for he is a child. But solid food is for the mature, for those who have their faculties trained by practice to distinguish good from evil.
Our understanding is "trained" by time and experience and testing and overcoming. This is reflected in the hard work in studying to show ourselves approved (2 Tim 2:15). Adam and Eve had no business eating the solid food of the tree of knowledge of good and evil.
How many times have wee seen destruction and disillusionment when a young or immature believer takes a doctrine he has not tested and goes about battering others with it. Or he imposes it upon himself which can lead to either slavery or pride. How many have abandoned the faith because of confusion caused by "system" teachers and preachers who doled out contradictory or confusing doctrines because the "system" says so?
Our humble and obedient and patient Lord showed us the way.
The Lord was offered the kingdoms of the world by Satan which one day shall be his. He obeyed the will of the Father and waited. The Lord knows good and evil, yet he discerns them correctly. Adam and Eve became as God that day, in that they became acquainted will good and evil, but they had chosen evil. This is why they had to have access to the Tree of Life forbidden.
The knowledge of good and evil does not, in itself, make one evil.
And the Lord God said, Behold, the man is become as one of us, to know good and evil...
Let's put these two thoughts together. The knowledge itself is not evil or wrong and it is also the distinguishing characteristic of the mature believer
But solid food is for the mature, for those who have their faculties trained by practice to distinguish good from evil.
Let us also note that wisdom and knowledge of both that which is good and that which is evil, it a characteristic of angelic beings.
As an angel of God, so is my lord the king to discern good and bad... My lord is wise, according to the wisdom of an angel of God, to know all things that are in the earth
-II Samuel 14:17, 20
Charles Welch notes:
There is a tendency with most of us to read the words of Genesis 2 as though they emphasized evil, the passage reading: “the knowledge of good and evil”, or even “the knowledge of evil”. We must remember, however, that the tree represented both good as well as evil. “Good” out of place, and before its time, can be definitely harmful. Marriage, for example, is “honourable in all” but that which is most blessed within the limitation of the marriage bond, is itself a sin if entered into apart from those Divinely appointed limits. Again we observe that “good” and “evil” are not things in themselves, but terms which refer to the actions of particular people.