A.T. Robertson Commentary Romans 13:4

"for he is a minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is a minister of God, an avenger for wrath to him that doeth evil." — Romans 13:4 (ASV)

A minister of God (θεου διακονος). General sense of διακονος. Of course even Nero was God's minister "to thee (σο ethical dative) for good (εις το αγαθον, for the good)." That is the ideal, the goal.

Beareth (φορε). Present active indicative of φορεω, old frequentative form of φερω, to bear, to wear.

But if thou do (εαν δε ποιηις). Condition of third class, εαν and present active subjunctive of ποιεω, "if thou continue to do."

Sword (μαχαιραν). Symbol of authority as to-day policemen carry clubs or pistols. "The Emperor Trajan presented to a provincial governor on starting for his province, a dagger, with the words, 'For me. If I deserve it, in me'" (Vincent).

An avenger (εκδικος). Old adjective from εκ and δικη (right), "outside of penalty," unjust, then in later Greek "exacting penalty from one," in N.T. only here and 1 Thessalonians 4:6.