The role of the serpent at the tree in the middle of the garden seems straight-forward enough, it is the phallus symbol that expelled Adam and Even from the Garden of Eden. Yet, as much as the serpent and the apple can symbolize sexual energy, the serpent on the rod symbolize kundalini energy climbing up our spine. The serpent is not the enemy, what we do is.
Same with the different interpretation of the greatest story ever told, the serpent as being the instigator who asked us to eat from the Tree of Knowledge, and with it, we lost our instinctive CONNECTION with GOD. Knowledge is not the problem, the mind can be in contact with GOD; it is just that the ego has to get lost in the holy NOW to experience HER.
Maybe the lesson learned is merely that we shouldn't have eaten the apple in the middle of the garden, we should have let the serpent climb the trunk of the tree instead.