I stated before that mankind was made in the image of God, but in ignorance (innocence). That condition was called good by God, despite the claim I've made that it was not his ultimate aim, nor will it be our condition in eternity. Our abilities (like will and choice) were made complementary to his because He wanted mankind to live on His level as family and friends. Though he is the omnipotent God, scary on so many levels, his aim is to have us be one with him.
What does all this have to do with the sin nature? God alone is good--only he has what it takes to express Godlike attributes in harmony with his own will, with what he knows is right. Only he can manage those things which make up his image. The sin nature arose in mankind when Adam and Eve, despite having the breath of God (a living Spirit), exercised Godlike capacities (of will) in opposition to God. Sin is, in fact, the exertion of will against the will of God.
As a consequence, the breath of God departed (spiritual death), mankind was thereby separated from God, and whatever capacity pristine man had to walk in the will of God was lost irretrievably (from mankind's side of the equation, at least). Since then, we walk in dying flesh apart from God, godlike to some degree, but anything but like God. We possess some godlike capacities, but without the ability to harness them to "good". We do what we have an urge to do regardless of what God wants: some more, some less.
That is the essence of our sinful natures. Adam and Eve had their life degraded to that level, and at that level they reproduced what would become all the rest of us. They passed on their broken nature as sinners, because it was all they had to pass on. The machinery of our soul cannot function without God being in us and us being in agreement with him, our malfunction is sin.