God warned man not to eat of the tree of the knowledge of good/evil or else he would surely die. As a matter of fact, from Eve's statement we know that God actually told man not to even touch the tree of the knowledge of good/evil.
The tree of the knowledge of good/evil is responsible for human death, suffering, the fact that everything runs down (2nd law of thermodynamics), the fact that our bodies age, and the fact that we are evidently born with Adam/Eve's post-fall nature.
That's one dangerous tree.
Eating of the tree of the knowledge of good and evil was simply disobedience of God- because He said
not to eat of it. Disobedience of God= death for man or angels because God is the author of their very natures-of who they are- and any action outside of those natures, any sin, IOW, is an offense against both Him, nature, and their very own selves as well, even though they presumed they were bettering themselves by this disobedience.
Our job now is to learn what Adam & Eve didn't yet know-that they
weren't bettering themselves at all, that to disobey God is to separate oneself from the author of life and all that He would have for us- life and life abundantly. Man must acknowledge and trust His creator in an intimate relationship of love, in order to have the integrity he was meant to have. Jesus came to reconcile and restore this relationship which was shattered at the Fall, to restore
faith in God, first of all, that should never have been spurned and lost to begin with.
The reason the tree was called The Tree of the Knowledge of Good and Evil is because the result of disobedience of God is to already know the first evil: separation from His will, separation from
Him, where man takes over the role of determining good or evil for himself, becoming his own god, with evil (moral evil, i.e. sin), as often as not the result. To put it another way, the first evil man experienced, or
knew, was the very separation from God that his act of disobedience obtained for him, because God is the ultimate good and that goodness remains intact in creation to the extent that creation (us) remains aligned with His perfect will, under His control. The difference with man (and angels as well), is that God let that control be optional, i.e. He gave them free will. We continue to know good and evil everyday that we live in this world where man's will is allowed to reign for all practical purposes-for the time being at least.