A.T. Robertson Commentary 1 John 3:8

"he that doeth sin is of the devil; for the devil sinneth from the beginning. To this end was the Son of God manifested, that he might destroy the works of the devil." — 1 John 3:8 (ASV)

He that doeth sin (ο ποιων την αμαρτιαν). "He that keeps on doing sin" (the habit of sin).

Of the devil (εκ του διαβολου). In spiritual parentage as Jesus said of the Pharisees in Joh 8:44. When one acts like the devil he shows that he is not a true child of God.

Sinneth from the beginning (απ' αρχης αμαρτανε). Linear progressive present active indicative, "he has been sinning from the beginning" of his career as the devil. This is his normal life and those who imitate him become his spiritual children.

That he might destroy (ινα λυση). Purpose clause with ινα and the first aorist active subjunctive of λυω. This purpose (εις τουτο) Jesus had and has. There is eternal conflict, with final victory over Satan certain.