According to Christian ethical views other than Divine Command Theory, what exactly made Adam and Eve's eating the forbidden fruit evil?

It is abundantly clear that the eternal God had purpose in creating a universe which is a suitable habitat for humanity. It is thus abundantly clear that humanity is central to the purpose of the Creator, whose creation it is.

Abraham calls God 'the most High, the possessor of heaven and earth', Genesis 14:19 and the vision given by God, through angelic means, conveys the 'everlasting gospel' which is :

Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters. [Revelation 14:6]

All was made by, and belongs to, the Creator who envisaged it, purposed it, determined upon it and created it and fashioned it and completed it and sustains it.

It is His.

And whom He is, is right. For He is God and there is none other above Him. Nor can there be.

The lie of the serpentine spirit which approached Eve is that the knowledge of good and evil (which springs into existence once there is a substantial creation containing sentient beings within it) is not the way to Life.

Organic life, natural life, flesh and blood type life ; was breathed into Adam through his nostrils. The 'breath' of God surrounds the earth and is breathed into Adam with every inflation of his lungs.

Oxygen pervades his inner mucous membranes, suffuses his bloodstream, carried by haemoglobin in every single red blood cell.

The life is in the blood.

But that is just organic. What of Life - the spiritual living force that will worship, serve, love, adore and thrill in a relationship with God who is Spirit ?

The Tree of Life was in the midst of the garden.

But neither Adam nor Eve partook of it.

Rather, they disobeyed God's words. Chose the words of another spirit - a created spirit. A spirit who misrepresented the true righteousness of God who needs no outer, external, administrative, worded 'knowledge' to guide Him.

Nor does humanity.

Out of the heart man believeth - unto righteousness. Romans 10:10.

Partaking of Law, as a means of sustaining life, will result (and does result) in sin and death.

Breaking God's command was sinful because all humanity can now see, with hindsight, that the reason God gave for not partaking afflicts us all - we all die, because we all sin.

"The wages of sin is death" Christians have been told in Romans 6:23 (though the rest of that sentence states that the gift of God is life through Jesus Christ our Lord.) God had warned the first couple that they would die if they disobeyed his simple, clear instruction not to partake of one particular thing. God's warning was proven to be correct. The couple disobeyed, they partook, and they died. Because they could not pass on to their offspring what they did not have themselves, we likewise all sin and die.

The course of sin has consequences. The ethical principal in the Bible is, "You reap what you sow":

"For they have sown the wind, and they shall reap the whirlwind... Sow to yourselves in righteousness, reap in mercy... Ye have plowed wickedness, ye have reaped iniquity; ye have eaten the fruit of lies: because thou didst trust in thy way..." Hosea 8:7 & 10:12-13 K.K.V.

This is where 'evil' comes in. It came in before the couple sinned, with a deceiver setting out to ensnare the couple, to cause their downfall. He lied, the woman believed his lie, and Adam trusted in his way of following his wife's way. The deceiver was evil, and by heeding the deceiver's lie, the couple sinned. They did not sin by "touching" the forbidden tree - because God nowhere said not to touch it; Eve misunderstood after the deceiver began tempting her to eat what was forbidden. Eve exaggerated the command of God because merely touching the tree or its fruit was not going to be the problem. Partaking of what had been held out as a way of elevating their position was the evil lie that led to their deaths.

Adam and Eve ate the fruit of lies, and trusted in their own chosen way, as the prophet Hosea said. But they could not claim that they were never warned. Nor can we.

The law of God has a clear distinction, the Natural law and the Positive law. The natural law is influenced by ethics and morality, while the positive law does not have morality and ethics as its core. Natural law leads us to maximize the well-being of our neighbors and minimize their suffering. Murder is an example of this type of law. The Positive exists by the pure will of God. Baptism is an example of this type of law. There is nothing ethical or moral that distinguishes between water and earth, but God has chosen water to perform this rite. Other laws exist that are a combination of these, they are Natural with a Positive application. For example, the Lord's Day is a Natural law that asks us to set aside a day of the week to keep it holy, but the application is Positive, the last day (of the week) in the Old Covenant and the first in the New.

While this is an interesting question as a thought exercise, not eating the fruit of the tree was a Positive law from God. There is no ethical or moral difference in eating from this tree beyond the fact that God decreed it so. Eve, by eating from this tree, broke a Positive commandment and a Natural commandment by wishing to be like God, that is, to have other gods (herself), and Adam as well.