Life under the sin nature, and life under spiritual realm, are mutually exclusive. They do not crossover or co-mingle. They are separate and apart. You are either living under the control and influence of the sin nature, or you are living under the control and influence of the spiritual life.
Paul clarifies that, in this verse.
The word for servant, doulos, literally means slave or bondage. That slavery or bondage could be either involuntary, as in the case of an unbeliever, or it could be voluntary as in the case of the believer who is out of fellowship.
Nevertheless, when you are under the rule of the sin nature, you are not under the rule of righteousness or the spiritual life.
For unbelievers, or those who have not believed in Christ and therefore, are not saved, their life is governed by the laws of sin.
And as we have already studied, when you are under the law of sin, you have only two choices to look to, in order to get out from under that law of sin. The first choice, is to do everything yourself, namely works, but this is a false option, and only ends in failure.
The second option, is that of faith in Christ. This is the only option that does work, which gets you out from under the law of sin, and into the sphere of the law of Christ, which is grace.
As an unbeliever, you do not have the law of Christ working for you. You have to first believe in Christ, and therefore become a believer, or a Christian, as it were, in order to leave the law of sin, and then enter into the law of Christ, which is the law of righteousness.
As a believer, you are under the sphere of the law of Christ, with respect to salvation.
However, with respect to your functioning spiritual life, this same principle applies with respect to your being in fellowship, or out of fellowship.
As a believer, when you commit a sin, you are out of fellowship, and your spiritual life does not function as it should.
As a believer, when you confess your sin, then you are back in fellowship, and your spiritual life functions properly as it should.
Again, being in fellowship or out of fellowship, are mutually exclusive environments. You are either in one or the other, but never both at the same time.