The Romans had become slaves of righteousness, but they needed to be exhorted to continue. Instead, we want the habit of doing good. When you were slaves to sin, you were free from the control of righteousness. When we sin, it might look like we are free from outside control, but we are really enslaved. Sin produces death, and we do not want to serve that kind of master.
Sin has bad results.
It gives us what we deserve. New life in Christ, on the other hand, has wonderful results, in this life and in the future. Choose life, Paul says.