The Christian life begins with a death to sin. The Revision says, Who died to sin. That is correct. The Greek verb is in the past tense. A fact is referred to that occurred in the past.
Death is a separation.
When we severed our relation with sin, we died to it. So many of us as were baptized into Christ, etc. The fact that every follower of Christ has died to sin is shown by his baptism. All its symbolism points to death. But this baptism into Christ implies death, for it is a baptism into the death of Christ.