The infamous Nero actually had a good run initially. He took power in 54 A.D. and until 59 (the same year he had his mother killed), Rome enjoyed a mini-golden age, the best since the time of Augustus. That period of time was one remarkable enough to be named the quinquennium Neronis. But between 59 and the year of his death in the late sixties, Nero spun out of control, becoming one of the great megalomanaical pieces of work in history.
The reason this is important is because Paul probably wrote Romans in 57 A.D., which means (follow me closely here) that he wrote Romans 13 in 57 A.D. This was right in the middle of that initial period of Nero's reign when Seneca the philosopher still had his foot on the brake. After Seneca (and another advisor named Burrus, the prefect of the Praetorian Guard) were gone, Nero went around the bend completely.
Now the first Roman persecution of the Church began in 64 A.D., when Nero tried to shift blame for the fire of Rome from himself to the Christians. So this means that at the time the letter of Romans was written, Paul's statements about the role of the magistrate as God's deacon of justice were not risible at all. Of course Paul writes elsewhere in a way that indicate he was not going to be surprised at all when Nero took his dark turn, not to mention Nero being the one who would take Paul's life. We too often forget that when Paul was writing about the magistrate being the deacon of justice, he was writing about the same man who was going to murder him.
If the magistrate is appointed by God to reward the righteous and punish the wrongdoer, then this means the instructions Paul gives in Romans 13 must be specific, occasional, and provisional. He doesn't say "submit to the magistrate, no matter what." That would conflict with what Scripture teaches us elsewhere. He says to submit to the magistrate because of the good they do, that good including punishment of the bad guys. But what happens when everything flips around, and the magistrate (seven years later) starts punishing the good guys? Now what? If the master is away, and he tells all the servants to obey the steward, who was left in charge to guard the silver, what should they all do when it becomes apparent to them all that the steward is stealing all the silver? God's law is constant, but circumstances aren't.
This means that the specific instructions in this passage are not a fixed star by which we are to navigate throughout all human history, and were never intended to be. They are provisional and conditioned. But the conditions can change in different ways, requiring change from us in one of two directions. (Of course when we change in either of these directions, we must do so on the basis of scriptural guidance elsewhere.) One change is when the need for state violence starts to evaporate because the leaven of the gospel has spread throughout all society. The Bible promises us that this will happen in multiple places, and since the advent of Christ it has been happening. There will come a time when we will study war no more. God did not give the sword to the magistrate in perpetuity, no matter what.
The other change is less glorious, and is more likely to happen in the earlier eras of church history, before the kings have brought their honor and glory into the New Jerusalem. This is the scenario when the magistrate uses the sword to punish the righteous instead of the wicked, and it gets to such an egregious level that disobedience and even active resistance become necessary. The case for civil disobedience and civil resistance (including armed resistance) is a peculiar subset of this entire discussion of violence, and requires separate treatment. Suffice it to say that revolution against the established authorities is never permissible for Christians (a constant from Romans 13) -- which means either that what Ehud did was not okay at all, or it was something other than revolution.
The constant from Romans 13 is that God establishes civil authority. The particulars in our obedience vary according to circumstance. At the very least it should be clear that Paul's comments there are specific, occasional, and provisional, not abstract and general.