The basic idea of "slave" is an indication that you belong to God, that you are committed yourself to life lone service to God.
The basic idea of "saint" is that you are dedicated, consecrated, and st apart for God's use exclusively.
Either as a slave or saint we are bought with a price, the shed bllod of Jesus, and as a result we belong to God. As a purchased possession of God we live a life of dedicated service to God. As saints we chose to be slaves. It is a decision we make to spend our lives in service to God. The Word of God tells us that Jesus, who knew no sin was made to be sin for us that we might be made the righteousness of God through Him. Saint or slave, we own our life to Christ in obedient service to Him.