I think most commentators have understood "terror to good conduct" a little differently than you are understanding it. The phrase is φόβος τῷ ἀγαθῷ ἔργῳ (phobos tō agathō ergō) — literally, "fear [to] the good work". This is nonsensical English, so the ESV has used "terror", which works. I think the idea they intend convey, though, is made more explicit in the NASB (bold mine):
For rulers are not a cause of fear for good behavior, but for evil.
The fear/terror is the emotion experienced rather than the objective historical fact of what is being done. Douglas Moo’s (widely regarded) commentary supports this view and further points out that "good behavior" and "evil" are likely personifications. We could expand the translation: rulers are not a cause of fear for those who do good works.
This still requires a bit of a leap: rulers do sometimes do things that are apt to cause virtuous people to fear. James Denney’s (older, but still highly regarded) commentary on Romans solves this problem with the little phrase about the term translated fear: "φόβος, as in Isa. viii, 13". Quoting the ESV:
But the LORD of hosts, him you shall honor as holy. Let him be your fear, and let him be your dread.
Understood in this context, "not a cause of fear" in Rom. 13 assumes a realization of this exhortation. Those who do good are also those who submit to this view of reality. For them, fear of other authority is irrelevant.
This "idealization" view seems to have been taken by both Origen and Augustine who went on to assert that the praise promised in the latter part of the verse is praise from God (see Moo, p. 800, fn. 50). Moo explicitly disagrees with that view, pointing out the parallel with fear aroused by the secular ruler in the first part of the verse. His position is that Paul was thinking of a specific situation in ancient Rome. Paul is drawing out an axiom that is useful insofar as it is generally true, but it need not apply in every situation.
Douglas J. Moo, The Epistle to the Romans (NICNT. Grand Rapids: Eerdmans, 1996), 800.
J.D. Denney, The expositor’s Greek Testament. (Hodder & Stoughton, 1897) vol. 2, 696.