well, it gets into the idea that a bad species has to have some sort of toltarian regime, and with that a type of Stasi/gestapo/kgb system that Watch's the population, and to protect the system. We have that with the Romulans, Cardasians, and probably other species. and in latter shows, shown that there is some type of resistance movement, or that the people are repressed by there overreaching government. That the people are generally good, but there government sucks. If you take a look at our world history, you'll see that most of our history is a history of oppression. How many pharaoh's, Kings, Queens, Dukes, Emporors, etc. Only recently, With the US being one of the first, is democracy. Regular people leading, not royalty, or some other type of family rule. ( though it doesn't seem like it sometimes, with family's like Kennedy keep on getting elected forever as an example). And who's to say the world wouldn't go back to an opressive system in the future? Look at the Terran Empire. For the Romulans, this may be a "phase" in there history, maybe in the past they were a more democratic, open society, then something happened, maybe a plague, maybe famine, and the government took more control of the system for the duration, and as always happens, once a person/people get power, they are reluctant to give up that power. They may have taken control of the system during the emergency, and didn't give up control afterwards, and using past examples in our history, with the Gestapo, Stasi, created the Tal Shiar as a means to "Protect" the system. Listening in on the populace, making agitators "Disappear" Etc. After hundreds, maybe thousands of years, systems rose and fell, and this is only the current iteration of there system and given the natural progression of time.. that was interrupted by the Supernova, they might have gone to another system. Maybe like England where there monarchy was relegated to more "Honorary" position, and the senate would be like the House of Lords, and maybe the creation of a House of commons. So just saying, it doesn't have to a Mono Culture that has existed In the same way since there landing on Romulas.