With very few exceptions (none which lasted long) virtually all of the most-powerful states in history have been tyrannical oligarchies (often led by dictators).

The remarkably-advanced (ideologically) Roman Republic included legal entitlements, freedoms and protections not only for the ruling classes, but also the general population. Sadly the deeply-corrupt Roman Empire supplanted the noble Roman Republic (as copied in the Star Wars mythology) after only a few hundred years - and it degenerated into and remained a militant nepotistic oligarchy (Imperial) until it fragmented and evaporated after about 1500 years.  

People who make comparisons of the US government (as formed by the Founders) as being supposedly, "based on the corrupt Roman Empire," are clearly ignorant of the fact that the Founders studied the Roman REPUBLIC, not the Roman EMPIRE for inspiration in writing the founding documents. Hence the vital difference between a pure Democracy (Roman mob rule) and a (Constitutional) Republic (rule by law, not people). 

There has been a shift in the attitude taken toward society as being governed by the Rule of Law. Originally in ancient times, laws were focused on being guidelines for justice with the objective of the law (justice) using legalisms (precise stipulations) as a tool to achieve justice. 

Largely and increasingly we have seen how focus on legalism has supplanted focus on justice. Instead of using law to achieve justice, "lawyers/judges" are soley concerned with whether legalisms are being conformed to. So conforming to the means (legalism) has become more-important than achieving the end (justice). 

The truism , "Neither the Nazis or the Communists ever broke a single law" is actually true. Because their states could change laws according to the whims of whoever was in power (they were not authentic republics but instead oligarchies) they never had to violate their own laws if they wanted to do something horrible. All Hitler had to do was change the laws and voila! Concentration camps were set up and genocide could proceed as "needed" by the German government. 

Of course, international laws were broken by the Nazis and the Communists (and the Ottoman Empire, etc.) in their time, but none of the atrocities they committed were against their OWN laws, because their sleazy relationship with republicanism enabled them to simply change laws at will. 

The essential difference between republican and democratic rule is this single element - when government changes hands in a republican form of government, the prevailing body of laws (which the population agrees is legitimate) does not change. So, in a republic, a new government does not mean a new set of laws - to achieve that sort of horror, once must govern by pure democracy (mob rule).