A.T. Robertson Commentary John 8:34

"Jesus answered them, Verily, verily, I say unto you, Every one that committeth sin is the bondservant of sin." — John 8:34 (ASV)

Every one that committeth sin is the bondservant of sin (πας ο ποιων την αμαρτιαν δουλος εστιν [της αμαρτιασ]). The Western class omits της αμαρτιας (sin), but that is the idea anyhow. Note the use of ποιων (present active participle, continuous habit or practice), not ποιησας (aorist active participle for single act), precisely as in 1 John 3:4–8. Note also 3:21 for ο ποιων την αληθειαν (the one who practises the truth). Sin, like the worst narcotic, is habit forming. Hence the problem today for criminologists for paroled or pardoned criminals nearly always go back to crime, sink again into sin, the slaves of sin. Xenophon has this notion of the slavery of sin (Memor. IV. 5. 3). So Paul clearly in Ro 6:17,20 "slaves of sin" (δουλο της αμαρτιας).