Jamieson, Fausset and Brown on John 8:34-35

34-35. Whosoever committeth sin--that is, liveth in the commission of it--(Compare 1Jo 3:8; Mt 7:23).

is the servant of sin--that is, the bond-servant, or slave of it; for the question is not about free service, but who are in bondage. (Compare 2Pe 2:19; Re 6:16). The great truth here expressed was not unknown to heathen moralists; but it was applied only to vice, for they were total strangers to what in revealed religion is called sin. The thought of slaves and freemen in the house suggests to our Lord a wider idea.