I just finished re-reading Paul Johnson's Modern Times. According to Johnson, moral relativism was the root of all evil in the twentieth century. I'm tempted to agree, but ultimately I doubt that meta-ethics played more than a supporting role in the statist horrors that Johnson catalogs.
Consider: If you're pushing mass murder, what's the best way to sell it?
Sales Pitch A: "Mass murder is good, here's why, let's go for it!"
Sales Pitch B: "Mass murder seems wrong, but morality is relative, so it's not really wrong. So let's go for it!"
The obvious answer, I submit, is Pitch A. And when you read Lenin, Hitler, and various other totalitarians who made it to the top, Pitch A is their main focus. Pitch B usually makes an appearance, but it's too confusing and defensive to win hearts and minds.
The main rhetorical purposes of moral relativism are to (a) reduce the guilt of your fence-straddling sympathizers, and (b) hastily respond to critics who point out how evil you are. And frankly, neither of these two functions seems to have been crucial for twentieth-century statism. Fence-straddlers aren't much help - and usually make up their own excuses for you, anyway. And when totalitarian movements face criticism, they get much better mileage out of ad hominem arguments, flat-out denial, threats, and the Big Lie than they get out of meta-ethics.
So what was the root of all (or at least most) evil in the twentieth century? Utilitarianism is another tempting candidate. After all, the theory does imply that murdering six million people to save the world is OK. But to be fair, utilitarianism only recommends mass murder in extreme situations. The scary thing about the twentieth century was that so many mass murders happened on the flimsiest pretexts.
So where did the twentieth century really go wrong? I'm not sure, but here are my two favorite candidates:
1. Tribalism. Yes, tribalism has always been with us. But in the twentieth century, tribalism became especially nasty. The most poisonous ideologies of the twentieth century - Marxism, Nazism, etc. - actually declared a large fraction of humanity to be sub-human. These sub-humans didn't just count for less than "real" people; the sub-humans had negative value. As a result, adherents of these ideologies were actually willing to sacrifice the lives of their own group to eliminate the bourgeoisie, Jews, kulaks, and so on.
2. The inverted "what is not seen" fallacy. In the twentieth century, people became bizarrely eager to believe that short-run horrors would lead to massive long-run benefits. A little judicious skepticism about "better tomorrows" would have taken most of the wind out of the sails of Marxism, Nazism, "national liberation" movements, and much more. In earlier times, people would have taken their empty promises much less seriously.