A martyr is someone who is a witness (“martyr” in Greek) to the faith, who is killed in hatred of the Faith (in Latin, “in odium fidei“) or in some cases, in hatred of the Church (“in odium Ecclesiae” – added by Pope St. John Paul II for clarity). Odium fidei can have a broad definition. (Also, if somebody dies as a result of religious persecution, even if the persecutor didn’t mean to go quite that far, the person goes from being a “confessor” (someone who stays Catholic despite persecution and suffering) to being classified as a martyr.)
So let’s list some martyrs who had similar situations to St. Oscar Romero, to show that it’s not something new.
St. Thomas a Becket, Bishop of Canterbury, martyr. Almost a one to one correspondence with the death of St. Oscar Romero. His attackers didn’t ask him to stop being Christian; they just asked if he was the archbishop. He was killed by political enemies who were also Catholic, because they hated him teaching Church teaching instead of going along with King Henry II.
St. Desiderius, Bishop of Vienne, was stoned to death by three assassins at the orders of a Christian king, King Theuderic II of Burgundy. He was martyred for preaching a sermon on chastity in the presence of Theuderic and his mom, the notorious murderer Queen Brunehild. (She was Christian, too. Fought with a lot of saints.)
St. Leodegar (aka St. Leger), Bishop of Autun and martyr. He was captured in 675 in Autun by the Christian Duke of Champagne and the Bishops of Chalon-sur-Marne and of Valence, and had his eyes gouged out and his tongue cut out. In 679, he was falsely accused of having assassinated King Childeric, was given a mock trial and exiled to Normandy, and then was led out into the forest there and murdered on orders of Ebroin, Neustria’s Mayor of the Palace. All of this was political, because Ebroin wanted to be the only power in all of Neustria; but it was also done in hatred and disregard of the Church.
St. Praejectus and St. Amarinus were a bishop and an abbot on Ebroin’s good or neutral side (from doing free government work for him), but most famous for the good stuff they were doing in their own area. They got stabbed to death by one of Ebroin’s enemies, under the belief that they had agitated for the execution of the lord of Marseilles. Both were immediately venerated as martyrs.
St. Rumbold, a missions bishop who moved around a lot, was martyred near Mechelen/Malines by two guys whom he’d told to change their ways.
St. Foillan and his companions seem to have been deemed martyrs, just because they were killed by bandits while traveling back through the forest from saying Mass at the nun monastery of Nivelles. That’s probably a little bit loose of a definition, but all the miracles probably had something to do with it.
There are a blue ton of martyrs from France and Germany who were churchmen who defied or annoyed political leaders for various religious and political reasons, and thus were deemed prime targets for murder. Most of the martyrs killed for refusing to violate the Seal of the Confessional are of this type.
For example, St. John Nepomuk, priest and martyr, was killed by King Wenceslas IV’s orders (tied up and thrown into the river), probably because he wouldn’t reveal the content of the Queen’s confessions; but really we don’t know because the king didn’t say. (All the ensuing miracles were kinda a sign that God thought it was something big.)
St. John Sarkander, priest and martyr, was tortured and martyred for not revealing the contents of a local Catholic leader’s confessions, because the local Protestant lord was sure that the local Catholic leader had brought on a recent invasion of foreign Catholic troops, and that of course one chats about troops to one’s confessor. (The foreign troops actually spared the area and went away again, thanks to the priest leading a Eucharistic procession near them; which made the local lord more suspicious and not at all grateful.)
There’s probably more to say on this topic. But there are lots of examples that go with a broader definition of how one can get killed in odium fidei. A lot of these guys are pre-conciliar (local bishops and popular acclamation just decided, so the rules were looser) but some aren’t.
UPDATE: Basically, the newer rules were set up to avoid the idea that any Churchy person who got killed was a martyr, or that any Catholic who got killed by non-Catholics was a martyr. Kings and warriors who died in battle defending the faith weren’t martyrs; but if they were captured, and killed in odium fidei after the battle while unable to defend themselves, then they could be deemed martyrs.
But there was still a fair amount of leeway on the breadth of topics that murderers could hate, that could also be deemed “hatred of the faith.” Pretty much anything that’s important to Church teaching or to the freedom of the children of God seems to have been picked, at some point. That’s where the martyrs of charity come in, as I understand it. Jesus taught us to lay down our lives for each other. So a martyr doing that is witnessing to Church teaching, and the person who kills him must hate Jesus’ teaching. Some people want to stretch that to all the virtues, but again, that’s probably going a little too far.
However, it’s plain that a little bit of stretch is inherent to the tradition of the recognition of martyrs, so we shouldn’t be so newfangled about it as to be too rigid.
It’s also pretty clear that most persecutors throughout history are not Roman magistrates out to reduce paperwork by giving one a chance to apostatize. Islamic persecution specifically includes many situations when attackers are instructed by the Quran and ahadith not to give people a chance to convert to Islam, either because it’s too late or because it’s deemed inconvenient even to sort out Muslims from unbelievers. People who hate the faith in less formal ways are usually interested in killing, not asking questions.