It is certain that human beings have a propensity to do evil.
Humans have a propensity to sin, not to evil. There is a difference.
I do not understand what I do. For what I want to do I do not do, but what I hate I do. Rom.7:15
If Paul was evil he would have loved what he did as a sinner not hated it.
My understanding to date regarding the questions you raise in your original post:
Original Sin = the sin originally committed by Adam when he disobeyed the word of God regarding the tree in the garden.
This act caused him to die (spiritually). His spirit left his body and returned to the Lord. His body was then made corrupt, don't know exactly how but similar to how brain cells would die if it is deprived of oxygen. This corruption is what we term the sin nature and it resides in every cell of our body (including male sperm but not female ovum, hence the virgin birth).
but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members. Rom.7:23
It is, as it were, a law unto itself that is wholly antagonistic to God. It is this "law" that traps our minds (ie our inner being = soul) and prevents it from doing the good that we want to do. There is no way out for our soul. No amount of will power will eradicate the sin nature within and it's influence over our soul.
This is coupled with the fact that without a human spirit we cannot know spiritual matters and as God is Spirit, that means all things relating to God cannot be known by us.
the sinful mind is hostile to God. It does not submit to God's law, nor can it do so. Rom.8:7
The "sinful mind" is the soul under the dominance of the sin nature that resides in our flesh and in an unbeliever (which we all once were) is a person without a human spirit which is why Paul says that person cannot submit to God's law because they have no "compartment" (human spirit) with which to understand spiritual mattters.
We are sinners from birth because we genetically inherit the sin nature through the male line. Our soul which, God imputes at birth is neutral in terms of good and evil, our bodies are defunct and we have no human spirit. This is why we are condemned, we are not what God originally designed and we are without hope of ever re-creating ourselves so that we might have life.
Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: Rom.5:12
That a man (unbeliever) can come to see good is by the grace of God. If a man does good it is also by the grace of God for if he had not heard the word of God, ie "do not murder" and believed it to be true, then he would have no power to do good.
This is where free will comes into the picture. Apart from grace, no mans will is free to chose and do good. The sin nature ensures this propensity and eventually it would transform our souls into an evil thing. This is my understanding of the doctrine of total depravity. Not that we are born evil but rather because of the sin nature within and no human spirit to know God we would ultimately slide downhill to the very depths of evil and we would be completely unaware that it is even happening to us.
The darkness into which we are born would be all we ever know and we would have no choice in the matter. All we could ever do is continue to feed upon the darkness in which we found ourselves.
However, "In the beginning was the Word", and Christ is the Lamb slain from the foundation of the earth. He took the responsibility upon Himself from the beginning that we should have a choice to choose life or death. He does not cause us to believe Him but He causes us to see His truth beginning with His sovereignity and deity.
since what may be known about God is plain to them, because God has made it plain to them. 20For since the creation of the world God's invisible qualitieshis eternal power and divine naturehave been clearly seen, being understood from what has been made, so that men are without excuse. Rom.1:19&20
To those who respond positively? these ones are drawn further into His truth.
"No one can come to me unless the Father who sent me draws him, and I will raise him up at the last day. Jn.6:44
How far a person will be drawn, only the Lord knows. However, the way in which He causes us to see His truth along the way is by His grace which is the greater power to temporarily disable the power of the sin nature within (in a manner of speaking) so our will is made free for a moment and we might make a choice according to our own desires.
The Lord is able to do this because:
1. He is merciful and wants all men to come to know the truth and..
2. Because Christ is the Lamb that was slain prior to sin.
Hence, sin is not the primary issue and in a sense never has been. The issue of sin the Lord took care of Himself in Christ. The issue for humanity is "Who do you say I am?"
"But what about you?" he asked. "Who do you say I am?" Matt.16:15
In the garden? Adam should have believed what the Lord said about the tree simply because it was the Lord who said it. He should have obeyed because he believed the Lord but he chose not to believe.
This is why the righteous will live by faith because faith is in the One who is true.
peace