I don't know how many Native Americans were killed by the U.S. government and/or citizens, but let's just make up a fictitious number for this question: two million.
Next, let's assume the Soviet Union killed five million of its own citizens. It's hard to characterize the slaughter as genocide, because a wide variety of peoples were targeted. Many believe the killings were motivated largely by a desire to wipe out resistance to the new government. There may have also been many revenge killings.
So, if we make a comparison based on numbers only, the Soviet experiment is the greater evil (based on my fictitious numbers; the actual numbers, of course, are far different). However, John Doe argues that the U.S. is the greater criminal because it committed genocide. He adds, "We can't blame socialism, because it's just an idea. Good people might make socialism work, while bad people will manipulate socialism (or any other form of government) to achieve more sinister goals."
To me, this sounds like a false comparison - comparing apples and oranges. The U.S. killings were driven largely by racism (which is obviously bad). The Soviet killings were driven by something else (which was also obviously bad).
If we want to dismiss the Soviet slaughter because it was done in the name of some inherently neutral political idea (socialism), then we should also dismiss the U.S. killings, because they were done in the name of another inherently neutral political idea (capitalism, democracy, etc.).
Vice versa, if we want to condemn the U.S. slaughter because it qualifies as genocide, then we should also condemn the Soviet slaughter, because it qualifies as state terrorism (or whatever). Or we could change our example to a country that slaughters five million women, or children. So now we're comparing racism and sexism, which is arguably just as bad.
Is there a name for this fallacy? To me it looks really similar to comparing apples and oranges. It might possibly be described as a bait-and-switch technique.
(For the purpose of this question, I'm not suggesting that either the U.S. or USSR is worse than the other. Rather, I'm focusing on the logic used to establish their "relative evil.")