|My thoughts on the concept of "Nachash"
When Adam and Chavah ate of the Tree of Knowledge of Good and Evil, the good and evil intermingled - mankind acquired within good AND evil (yetzer hatov and yetzer harah - the good and evil inclinations). Evil was no longer concentrated in a single outside source - the "nachash" (serpent). Good and evil would be completely intermingled. Now mankind would spend all his days fighting his evil inclination and attempt to overcome it. Adam and Chavah, who I believe did not have the yetzer rah, instead having only inclination for good, developed/internalize the yetzer rah by sinning, which they were deceived/tricked into. Whereas, mankind today is born with the yetzer rah as a part of our nature at birth and we can either strengthen it or strengthen our yetzer tov to overcome it - solely our choice (bechira)
Adam and Chavah had an external yetzer rah - the nachash, which was cunning and confused their rational thinking. I don't believe they "chose" to follow one inclination over another, as is the case mankind does now, because they were created with an inclination only for good. The nachash confused Chavah with truth, she took literally the mitzvah she herself added in stating, "But from the fruit of the tree that is in the middle of the Garden, G-d said, 'You shall not eat from it, neither shall you touch it, lest you die.'" (Bereishit 3:3)
G-d had not said this. He said, "...but from the Tree of Knowledge of what is good and evil, you shall not eat from it, for on the day you eat from it, you will certainly die." (2:17)
Chavah states the mitzvah to not eat of the Tree but then adds a fence around that mitzvah - don't even touch it, therefore one won't be tempted to eat from it. She placed the fence above the original mitzvah. It is common knowledge in Judaism that down through the ages the fences that have been placed to protect and secure Torah observance does not override the original Torah mitzvah.
The nachash knew of the addition she had added and used it against Chavah, deceiving her into breaking the mitzvah of G-d. The Torah writes that the nachash said to her, "You certainly will not die." (3:4) The Sages teach that the nachash then pushed Chavah against the Tree and seeing that she certainly did not die, she rationalized that eating from it would not harm her either - therefore she was cunningly deceived by the nachash.
We "choose" to follow one inclination over the other, we are not tricked into it. The way of overcoming one's evil inclination is through the remedy given by G-d - the Torah. Just as Gan Eden (Garden of Eden) had flowing out of it a river symbolizing a link between G-d and the world outside represented by this river. That even though mankind was expelled from Gan Eden, a link remained. So too, even though we may turn after our evil inclination, there remains always a link back to G-d - the Torah is that link.
G-d said, "Let us make man in our image, after our likeness..." (1:26) In G-d's image - with the ability to reason. "And thus G-d created man in His form. In the form of G-d, He created him, male and female, He created them." (1:27)
The Hebrew word "tzelem" (image) denotes appearance, while the Hebrew word "demut" (likeness) denotes similarity in form and deed. Since G-d does not have form it is believed this is meaning the image which was seen by the prophets when G-d revealed Himself to them. Rashi explains that Scripture is referring to man's intellect, which is somewhat akin to that of the angels. Angels do not have a yetzer rah, an inclination for evil. Therefore, I do not believe that G-d has an inclination for evil and neither would have Adam when first created. As Midrash Haneelam Zohar Hadash 16 says, "'In our image, after our likeness'...For even as the soul which came from Me holds aloof from mundane matters, so will he in body yearn and aspire to the lofty and holy."
After the fall of Adam and Chavah mankind was forced to seek to return to being in the image and likeness of G-d. Adam and Chavah had body/garments light (ohr) as the Tikkune Zohar says, "And G-d created man in His own image, which means in the likeness of the Shekinah,"
We possess garments of skin ('ohr) with the qualities, mentally and morally, to be in this image and likeness of G-d, not created so as Adam and Chavah, but through observance of the mitzvot, the divine precepts, Torah study and the meditation of G-d's Word, which causes tikkun (repair), a complete restoration of ourselves and return to what we were meant to be. This is why RaMBaM says that in the Olam Habah (World to Come) we will be without the yetzer rah and thus will have no need for bechira chofshit - free will/choice. I believe we are 'born' with bechira chofshit, but I do not believe that Adam was created with bechira chofshit. Instead I believe Adam abided by the Word of Hashem as a part of the nature created in him ("created in the image of G-d"), naturally doing what is good. Similar to what the RaMBaN writes, that in the times of Mashiach there will not be bechira chofshit. Doing good will be our nature.
When we 'overcome' the nachash (yetzer harah), with the help of Hashem, instead of being subject to it, it then becomes subject to us and we stop looking for ways around the Torah - "Hashem said to Kayin, "Why are you angry? Why are you depressed? Is this not so - if you improve, there is forgiveness, but if you do not improve, sin rests at the entrance. Its desire is unto you, but you can overcome it." I don't believe that Adam had this to overcome until he fell from his natural inclination for good.
I also believe that ha-satan is the same as the yetzer harah which is the same as the nachash, just different aspects of it or names for it. Nachash, which was physical (external) in Gan Eden was the yetzer rah that was in Gan Eden and became internalized which later became known and called hasatan, the adversary. Any aspect that causes us to incline towards not doing the Will of Hashem is an adversary whether it be internal or external.
G-d created Adam and Chavah with the ability to reason of which the nachash took advantage of. It "reasoned" with Chavah with the truth and confused/tricked her into eating of the tree - "the nachash was more cunning than any beast of the field..." (3:1) She "seen" things differently than before his deception, she sees the fruit was good for food, a beautiful fruit and that it would give her wisdom (3:6). The nachash confused Chavah, "The nachash deceived me, and I ate." (3:13) The nachash possessed the power ("cunning") to create a conflict between her reasoning (thinking) and her emotion (feeling). Her emotional attraction was powerful enough to overcome her reasoning caused by an external stimulus which caused the conflict - the nachash. After Gan Eden, in mankind this stimulus is internalized as the yetzer rah - evil inclination.