Paul said that he did what he hated and that as long as he knew the law of God was good, that shows it's the sin in him and not him that does it. But he also said if your sinful nature is controlling you, it leads to death. If you have God's Spirit, you won't have your sinful nature controlling you. What does a Christian do when they are totally and completely controlled by their sinful nature? Paul said such people do not have God's Spirit in them. People in their sinful nature think about how to please the flesh, but people in the spirit think about ways to please the Spirit. I hardly ever think about how to please the spirit; all I do is wish that I could and wasn't in the shape that I'm in. I fear death, and if I fear death, doesn't that mean I have good reason too?
Let's start with the last question. Just because a person has a fear, it doesn't mean the fear is reasonable. There are people who have irrational fears or fears that are taken to an extreme. A person who is living in sin, continuing to do wrong while knowing it is wrong, ought to fear death. A person who knows he is living right to the best of his ability, and who makes corrections to his life when he realizes he is doing something wrong, should have no reason to fear death.
Before getting into your question, I need to straighten out a concept. "Sinful nature" is not a concept taught in the Bible. The New International Version uses "sinful nature" where the Bible says "flesh," but it is a poor translation at best because people walk away with the wrong ideas about what is being said. See "Since man is created in God's image, how can he have a sinful nature?" for details.
What you are referring to is:
"For we know that the Law is spiritual, but I am of flesh, sold into bondage to sin. For what I am doing, I do not understand; for I am not practicing what I would like to do, but I am doing the very thing I hate. But if I do the very thing I do not want to do, I agree with the Law, confessing that the Law is good. So now, no longer am I the one doing it, but sin which dwells in me. For I know that nothing good dwells in me, that is, in my flesh; for the willing is present in me, but the doing of the good is not. For the good that I want, I do not do, but I practice the very evil that I do not want. But if I am doing the very thing I do not want, I am no longer the one doing it, but sin which dwells in me. I find then the principle that evil is present in me, the one who wants to do good. For I joyfully concur with the law of God in the inner man, but I see a different law in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin which is in my members. Wretched man that I am! Who will set me free from the body of this death? Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin" (Romans 7:14-25).
Paul describes a dilemma in his life that is familiar to everyone who desires to live righteously. It is one thing to know what you ought to do and something else to practice it consistently. As a Jew living under the Old Law, Paul knew what the law required of him. Rationally he knew that God's requirements were good for him and it was his desire to do God's will. The problem was that he had weaknesses that originated from living in this physical world.
Let me illustrate it this way: you are walking along the road when suddenly your friend pushes you down. You instinctive reaction is to be furious with you friend, up until you realize that he pushed you out of the away of an oncoming car that you had not noticed. The instincts in your body push you in one direction while your mind knows that in this case the instinctive response is wrong.
That is how Satan lures us into sin. He capitalizes on our desires, our instinctive reactions, to pull us into situations that are sinful. "But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death" (James 1:14-15). In our minds, if we stopped to think about it long enough, we realize that what our bodies want to do at the moment isn't good, but too often we don't pause to think.
This is what Paul was saying. Instinct isn't able to make moral choices. Our bodies don't know the difference between right and wrong. So because our minds, at least at times, rebels against doing wrong, our minds agree with the law and righteousness. We want to do what is right, but too often lack the strength of will do follow that course. The Law condemns sin, so we find ourselves doing wrong, agreeing that is wrong, and agreeing that we stand condemned.
If matters stayed like that we would all be lost because even the best of us know there have been times we did not do what we should have done. That is where Jesus comes in. He gives us a way out of the bondage of sin. "My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous" (I John 2:1).
There are people, however, who do not care about living righteously. They follow their instincts regardless of whether it leads them true or into sin. Christians do care. Their goal is to live righteously.
"For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. For the mind set on the flesh is death, but the mind set on the Spirit is life and peace, because the mind set on the flesh is hostile toward God; for it does not subject itself to the law of God, for it is not even able to do so, and those who are in the flesh cannot please God" (Romans 8:5-8).
A person who lives for this world only, whose focus is to go wherever instinct takes them, will never please God. You can't accidentally live righteously. The person might do righteous things once in a while, but those occasional good deeds doesn't define him. So in the end he will spiritually die. However, the Christian is focused on doing what is right. He may sin once in a while, but those occasional straying into sin don't define him. He refuses to stay in sin. "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:8-9).
In other words, you cannot be a Christian and be totally controlled by sin. When a person chooses to be a Christian, they are no longer slaves to sin. "But God be thanked that though you were slaves of sin, yet you obeyed from the heart that form of doctrine to which you were delivered. And having been set free from sin, you became slaves of righteousness" (Romans 6:17-18).
Thus, the question isn't what you have done, but where you are going. Which path are you going to follow?