A.T. Robertson Commentary 1 John 3:4

"Every one that doeth sin doeth also lawlessness; and sin is lawlessness." — 1 John 3:4 (ASV)

Sin is lawlessness (η αμαρτια εστιν η ανομια). The article with both subject and predicate makes them coextensive and so interchangeable. Doing sin is the converse of doing righteousness (2:29). The present active participle (ποιων) means the habit of doing sin.