The problem is that power attracts evil people. If you are a murderous psychopath, better to be the one making the laws than breaking them.
In just about the last century alone, the following countries have suffered genocide: Germany, Soviet Union, Poland, Ukraine, Czechoslovakia, Yugoslavia (Bosnia, Croatia, Serbia), China, Cambodia, Vietnam, North Korea, Indonesia, Bangladesh, Iraq, Armenia, Darfur, Burundi, Rwanda, Somalia, East Timor, Ethiopia, Guatemala, Greece, France, Belgium, Netherlands, Denmark, Austria, Hungary, Romania, and Bulgaria.
And unfortunately, that list is surely incomplete. It also does not include war; genocide is the systematic massacre of civilians, generally of a particular ethnic, religious, or political class. Every one of these genocides has been carried out by a government, or with the government supplying, backing, or supporting the mass murder.
This is devastating to think about, but an important perspective. Governments are very dangerous. And it is crucial to remember this even in the best of times. It is the reason even the most benevolent government should never be given even the slightest power to ignore individual rights.
Most genocides have decades-long build-ups, they don't simply happen overnight. A disarmed populace could wait quietly for a quarter century before being violently attacked. When the genocide comes, the people in government may be entirely different than the policymakers who disarmed the people.
This is the problem with collectivism, with mob rule. The winds can turn violently against a minority at any time. The minority is not always ethnic–in Cambodia, the educated classes were slaughtered. Wearing glasses made you a target. Ukraine was targeted by Stalin because they were least supportive of the Soviet Union. Famines were orchestrated to suppress and murder political opponents.
The genocides in Rwanda and Burundi were between the Hutus and Tutsis. They were not rival tribes or ethnically different peoples. Belgian colonials separated the Hutus and Tutsis by cosmetic characteristics. Shorter, thicker, darker people were labeled Hutu, and taller, lankier, lighter skinned people were labeled Tutsi. Tensions would boil for over 50 years before escalating to genocide.
Genocides in places like Darfur and North Korea are ongoing. Other places like Venezuela and Nicaragua are fomenting tragedy.
At least 135 Nicaraguans have been killed by government forces since April. Protesters have taken to the streets after third-term President Daniel Ortega has come down increasingly hard on the political opposition. There is some evidence that killings, torture, and other human rights abuses have been authorized by the highest level of government.