The temptation is about abusing freewill instead of relying on God's Word. Satan is tempting them to choose what is good and evil for themselves and not rely on what God should propose is right and wrong.So to answer your question as to why man should have to die to come to the knowledge of good and evil: because it is sin. It is pride, it is selfishness, it is blasphemy to assert that man (in any state whether "perfect" or fallen) is capable of matching wits with God. God knows of an eternal perspective of what is right and wrong, and man does not, since none other can have a “God’s eye view,” to know the end from the beginning. (Isa 46:8-10). The choice of life is to receive God’s Word as truth, in faith, or to go blindly through life making choices for temporal and immediate satisfaction, never knowing the latter end of the consequences of our choices until they catch up to us (Matt 15:10-20)!All that Adam and Eve knew was created upon the foundation that God's Word is true (Psalm 33:6, Heb 6:16-20, Isa 55:8-11). All that they knew of God was grace. They were created by Him, and appointed as ruler over His works, and set in His garden.Satan came professing that God was denying them something by the boundary of His commandment and that they should experience for themselves what they should consider to be good or evil and not be lead about by mere faith. He also insinuates a manner of protection from the fact that God, who cannot lie or contradict Himself, gave them EVERY tree of the garden and blessed them that they should multiply and replenish the earth (Gen 1:27-30). And the commandment coming afterward, cannot disannul the promise of the gift (Gal 3:15-18, Rom 7:4-13). Satan seeking to be honored as the “forerunner,” paving the way of this rebellion (Heb 6:19-20) as a very anti-type of Christ. And since all that God will’s comes to fruition, man should exercise his own will, and be “truly” like God, that he might have the things he considers good, and avoid only those things that he should consider evil! I find it heartbreaking that God calls to Adam seeking a confession and repentance and instead is blamed Himself for somehow allowing or forcing this to happen. And what does man do with this opportunity for forgiveness…the woman says the devil made me do it, and man says the woman who YOU made for me gave it to me (a false trinity in one accord sets out to sin, and in the testing of judgment turns one on another)! I also find it interesting that after they made aprons of leaves for themselves they say that they hid from God because they were naked…some might call this a contradiction, but I call it profound!God’s judgment is to remove the excess of His grace (which Adam and Eve already abandoned since they were willing to die to disobey God), to test the power of their own will (Job 40:1-14). Surely, man, that would take all of God’s grace in vain, only to throw it away to make a name for themselves, commemorating their own accomplishments, can attain unto such a state that God gave them freely by His grace, that they should not die…NOT!The reason God’s judgment seems so harsh is because our hearts are callous against sin (Eze 11:19-21, Joel 2:13, Deut 10:13-22). This act of disobedience made necessary the sacrifice of Christ. Because “there is no other name by which man can be saved,” the act of sin, forced the act of the cross. Not the eating of fruit, but the partaking of sin made necessary the death of Christ (the cross is the very symbol of sin, the broken Word of God, seeing He was made sin for us, and His body broken for us. 2 Cor 5:21, 1 Cor 11:24), and therefore in judgment makes necessary the death of Adam and Eve who refused repentance and therefore accept the consequence of their actions! That godly nature in which they were created being abandoned and left upon that “tree” in the moment of the WORKS of their apostasy, just as Christ, the very image and likeness of God, “hung upon a tree” in the act of sacrifice, providing a means of our redemption, by which we receive through FAITH (Gal 3:6-14, Eph 2:8-22).For our failure in sin is reverencing either our own judgments or even God’s gifts (whether by the law or grace) above a relationship with Him. In sin, Adam and Eve forsook all of God’s graces, in faith, upon the word of Satan (Refusing the Word of God and honoring Satan’s words, thereby yielding themselves servants to their new master and god. (Rom 6:16-23); surely than, we are to forsake the mere creature comforts of this world, and bear our cross, having been redeemed by the righteous Word of God and the true Angel of Light! For the proof of Jesus Christ being our only means of salvation and glory is spoken by the very mouth of the LORD, “Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live forever.” For Adam and Eve were surrounded by the world of God’s grace, and hearkened unto one voice of temptation; therefore redemption shall be to those surrounded by a world of sin, among a people defiled by their own works, who hearken to the words of God’s true Angel of Light, sent to redeem all things unto Himself! As the Son of God, Christ is held up as a righteous contrast to Adam, who betrayed the Father and led all people through himself toward sin. This far superior glory in Jesus is revealed manifold in His coming in the flesh, As the Son of Man (Adam); avenging Adam’s betrayal of humbling himself to the ways of sin, through Christ humbling Himself to the death of the cross, manifesting the gospel of grace; whereby all in Him are redeemed unto righteousness and holiness unto the LORD!!! For if Adam, a Son of God, can betray his heritage and nature in God, to rebel and lead all within him unto sin, then Christ, as the Son of Man, contrary to the fallen nature of the flesh, can sanctify unto righteousness all in Him through the gospel of grace (Rom 5)!!!