Brutal mass-murdering dictators may have different philosophies and methods, but one thing they universally have in common: They strip the masses of their means of self defense.
Firearm regulations already existed in Germany when Hitler came into power. However, Hitler's government passed the 1938 Regulations Against Jews' Possession of Weapons, stripping from Jews the right to possess weapons of any kind, and thus ensuring minimal resistance to the Nazi programme of Jewish genocide [ref, ref].
In 1938, Mao Zedong declared that "political power grows out of the barrel of a gun". In 1957, under the pretense of "safety provisions", China outlawed the private manufacture, repair, purchase or possession of all firearms and ammunition.
Pol Pot’s Khmer Rouge immediately began disarming the civilian population, under the false-friendly pretense of protection. Once the disarming was complete, they proceeded with their genocide, killing approximately 1.5 million people in just under four years [ref, ref, ref].
Hitler, Stalin, Mao and Pol Pot are among the worst offenders, but they aren't alone. Millions of defenseless, disarmed people have been murdered by their governments in Armenia, Cuba, Guatemala, North Korea, Poland, Pakistan, Uganda, Vietnam, Yugoslavia and elsewhere [ref, ref].
You may argue that even if the average citizen was able to possess weapons, they wouldn't have been able to prevent the genocides. Perhaps not, but they certainly would have made it much more difficult for the despots to carry out their designs. I, for one, would rather perish resisting tyranny, and take as many of the despot's agents with me as possible.
As for the claim that "it could never happen here", I say hogwash. Probably the great majority of victims of genocide, along with those that escaped the genocide, believed that they were safe — until they weren't.
The price of liberty is eternal vigilance. You can't count on things always being good. Jews in Germany thought they were safe because Germany was a modern, enlightened, industrialized country where they had been safe for hundreds of years. In a short twenty years that turned around. Jews in Germany submitted to Nazi gun control laws and allowed themselves to be disarmed. And because they'd lost the will to fight, a third of the Jews on this planet were murdered. [Guns in America, by Jan E. Dizard, Robert M. Muth, Stephen P. Andrews; page 405]
How quickly circumstances can change, and fundamental rights, freedom and lives lost. Political power does, indeed, grow out of the barrel of a gun, the price of liberty is, indeed, eternal vigilance, and the guarantor of freedom is, indeed, as Thomas Jefferson taught, a well armed citizenry:
When governments fear the people, there is liberty. When the people fear the government, there is tyranny. The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.