Everyone who sins also commits lawlessness. Sin is lawlessness.
Sin stands in opposition to purity. Furthermore sin is very serious. The use of the Greek word translated "lawlessness" (anosmia) carries a connotation of wickedness (cf. Matthew 7:23; Matthew 13:41; Matthew 24:12; 2 Thessalonians 2:7). It means rejection of law in its broadest concept, flagrant opposition to God, rather than just breaking specific laws. Evidently the false teachers had a soft view of sin (cf. 1 John 3:7-8).