Extreme intolerance of any sort is evil. While the death toll from communist and fascist regimes are oft-cited examples, communism and fascism by no means have a monopoly on pointless mass killings. An estimated 100,000 executions of supposed communist sympathizers by South Korea occurred over a few weeks during the Korean War, despite South Korea being a supposedly democratic nation.
Even ignoring the fact that executing people for simply believing in communism is evil in itself, many of these executions were of people with no affiliation with communism in the first place. And such extreme intolerance of communism continued for a long time to a lesser degree even after the war ended. While probably not as bad as the intolerance shown by North Korea, this hardly makes it fine and dandy.
This is the kind of thing that happens when a government or populace is swept up in a frenzy of extreme intolerance, regardless of where their intolerance comes from. This happened during the French Revolution, which aimed for a democratic society, and between Catholic and Protestant Christians, where millions died due to religiously motivated conflict. Even twentieth-century United States has its moments of extreme intolerance with the Japanese-American internment camps and the oppression during the McCarthy era.
While many of us are fortunate enough to live in enlightened societies that are relatively tolerant, we should always beware of bigots that cannot tolerate those that are different from them, e.g. homophobes and sexists. And we should always strive to be tolerant ourselves, and not simply be smug that we are tolerant of others, lest we unknowingly become excessively intolerant without good reason.