You did almost affirm it with your idea that man has a "sin nature". Scripture does not actually use that term, but it does use "sin" as a "power" and as a "master" and as an enslaving force (my view).I have never affirmed this. It is the sort of result that you get when you start your theology with man instead of with God.. Sin is a falling short of the righteousness an holiness of God as I pointed out earlier.
The rejection of God is a moral transgression.
However, the idea of 'root' sins and 'fruit' sins is an interesting one. According to Richard Owen Roberts, there are, not one, but at least three root sins, of which unbelief is one. In Jude 5-7, we are confronted with three sets of sinners: The Israelites who 'could not enter in because of unbelief' (Hebrews 3:19); the fallen angels who rebelled against God, and the Sodomites whose root sin was pride (Ezekiel 16:45). I have heard others speak of selfishness as a root sin.
Indeed it is; the rest does not follow from that. It is like your faulty analogy of the man drowning. The reason he drowns is that his lungs become filled with water. The reason men are condemned is because they sin and come under judgement.
That said, I never said that the rejection of God was not a moral sin (it is not, by definition, a "transgression"....if we are taking our definition of "transgression" from Romans).
You say that sin is a moral transgression.
I say that sin is a moral transgression, and an enslaving principle. Why should I have to deny passages that speak of sin as such a power in order to affirm it's moral implications???? Why should I have to deny that those who are condemned are condemned because they do not believe in the name of the only begotten Son in order to affirm that the lost are "left in their sins"????
You do not have to deny one passage in order to affirm another. They all work together. This is my point. You do not have to limit the Atonement only to its judicial aspects because Scripture does not limit the Atonement only to its judicial aspects (Christ can mediate because He knows our suffering; Christ is the Head and we are the body; God was in Christ reconciling the World to Himself, etc).
Take a lesson from Spurgeon - let your answer not be "yes, nay" to Scripture but "yes, yes". Don't be Spurgeon's "Nelson".