If we are self-serving and destructive, we will end up tearing each other apart, but if we serve one another in love, we will build the church. After He redeemed us, God gave us great freedom of mind, action, and choice. He has freed us from the curse of the law—the death penalty. He has freed us from the fear of death, from enslavement to sin, and so on.
Then He says, once we are freed, we need to use this freedom to serve. This is where the idea of being a slave of righteousness enters the picture. He severed our relationship from our former master (sin, Satan, the world), freed us, and then took us into slavery to Himself and to serving our brethren in righteousness.
Of course, as Paul said here, this fulfills the intent of God's law: love, outgoing concern, the way of give.
Richard T. Ritenbaugh
It Takes a Church
Romans 8:9 says that we, as Christians, are not in union with the flesh but with the Spirit. Paul means us to understand that we exercise a power here—a control. Thus, he says here, "Make the right choice! Don't use your liberty, your power, to satisfy your flesh, but by love serve one another."
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 9)