Why compare evil regimes? Sometimes a choice must be made which one to support. Inaction and refusal to choose is also a choice and may favour one or another. The help or harm to some regime may be indirect, e.g. through the enemy of an enemy.
How to compare evil regimes? I have encountered people who compare based on words, justifying crimes against humanity by some countries with the argument that their goals were good or the ideology was good, just wrongly implemented. (The subtext here is that if it was wrongly implemented in the past, perhaps it should be tried again in the hopes of implementing it rightly.) I disagree. Actions should be the basis of judgement, not narratives. A failure of a political system is a negative signal about it. Regardless of whether it signals a fundamental flaw or a low likelihood of right implementation, until all other systems have been tried and have accumulated a similar weight of negative signals, the failed system should not be tried again. This can be mathematically formalized as optimal sequential control under incomplete information.
I believe comparisons of countries should be based on objective criteria, preferably specified before the data is gathered (as in the scientific method). These objective criteria are for example the number of people killed, tortured, wrongly imprisoned, expropriated, the number and extent of wars started, the territory and population conquered and for how long, the economic and environmental damage caused. The number of ethnic or religious groups eliminated may also be counted, but this has the effect of weighting the deaths of people from smaller groups more.
The measures can be total or divided by time or by the number of supporters of the regime. The total of these criteria is generally larger for bigger countries. There is simply more opportunity to kill, torture, etc when there are more people available. The total measures are of interest because they show the whole negative impact on the world.
Division by time results in criteria that measure the flow of evil done. If the decision is which regime to eliminate first, it is optimal to focus on the one with the greatest predicted negative influence per unit of time. This strategy minimizes the total impact of evil regimes.
To find the expected number of crimes of a person from a dictatorship (or its leadership) without other data about them, the total crimes of the regime should be divided by its population (or number of leaders). Dividing by both people and time gives the expected flow of evil per person, suggesting an optimal strategy of removing leaders of criminal regimes.
The above focusses on past evil, but for predictive purposes the attractiveness or “selling power” of the regime also matters. How likely is the dictatorship to survive and expand? If more people favour and justify it, including outside its borders, it has a greater opportunity to do evil in the future. So the niceness of the narrative used to excuse its actions is actually a negative signal about an otherwise criminal regime. If the stories the dictatorship tells about itself make people consider its goals good or ideology good, then the dictatorship is more dangerous than another that cannot manipulate audiences into supporting it.
Principles help in resisting the siren call of “the end justifies the means.” For example the principle that nothing justifies crimes against humanity. No story about the greater good, no idealistic ideology. Another good principle is that actions speak louder than words. If a regime fails at good governance, excuses should be ignored.