Or HaChaim on Genesis 3:4/אור החיים על בראשית ג׳:ד׳

1 ויאמר הנחש…לא מות תמותון. The serpent said: "You will most certainly not die." The reason the serpent repeated the reference to death was to counter Eve's fears that the tree itself was lethal, or that even if it were not harmful at all, G'd would punish them with death for disobeyimg Him. The serpent claimed that neither natural nor judicial death would result from contact with the tree or its fruit. Assuming that the serpent was right, this in turn raises the question of why G'd had forbidden the fruit of that tree? The serpent had to make such a prohibition sound plausible. This is why it continued that G'd was aware that as soon as man ate from that tree he would become a real competitor to G'd, i.e. knowing what is good and what is harmful. This argument was pure blasphemy. The only reason that the wicked serpent was able to employ such an argument was that its creation had preceded that of man. The serpent was able to use its senior status in the history of creation to claim that it was privy to matters that Adam and Eve could not have been privy to. He also argued that the reason he (the serpent) had been denied entry to גן עדן was that entry had been permitted only to those who were not privy to the secret of the tree of knowledge and what it represented. It claimed that by preventing the serpent from entering the garden G'd had wanted to preserve the secret of the power that tree would impart to those who ate from it. According to the serpent, once Eve would eat from that tree she would no longer be afraid of G'd, and G'd would no longer exercise any control over her. The use of such an argument was idolatrous. According to our halachah idolatry is punishable even when it is merely an intellectual conviction (Berachot 12).

2According to Bereshit Rabbah 19,3 there is another reason that the serpent used the expression מות תמות. The serpent pushed Eve against the tree and "proved" that touching the tree was not lethal- as Eve had claimed. It argued that just as touching the tree had not resulted in death, neither would eating from its fruit. We must examine this so-called "proof." If we accept that Eve had convinced herself that contact with the tree would have fatal consequences because of its very nature, the fact that her contact with the tree had proved harmless should have convinced her of the tree's harmlessness. There was therefore no reason to be afraid to eat from it. Alternatively, if Eve's fear of touching the tree were based on her belief that G'd had outlawed both eating from the tree and touching it, she now had "proof" that this was not so because nothing had happened to her after she had touched the tree. We do not consider this in the nature of any "proof," seeing that G'd's punishment did not have to be meted out immediately. We therefore prefer the explanation we offered previously that the serpent wanted to "prove" to Eve that the tree itself was quite harmless.

3If you were to ask why G'd would have created a tree that was poisonous at all, the answer is -as in many such instances- that the greater the physical and spiritual effort to overcome such potential impediments to our faith, the greater the reward which is stored up in heaven for such acts of faith, a reward to be consumed by us in the Hereafter. This concept is phrased in Avot 5,26: לפום צערא אגרא, as "the reward is commensurate with the effort expended." Had G'd imposed strict limits on the power of the seducer to entice us, overcoming him would not have amounted to much. The Zohar illustrates this principle by the example of a father who wanted to test his son's moral fibre and who instructed a beautiful harlot to use all her wiles to seduce him. At the same time he had instructed the son to keep his distance from the harlot in question.