It's a point that a lot of folks have made, but Matthew Yglesias makes it especially well:
Why do I keep hearing people point out that what went on at Abu Ghraib under U.S. command wasn't as bad as what Saddam did? Not that it was as bad, but to even raise the comparison bespeaks a very telling insecurity. Gerhard Schroder doesn't respond to criticisms of his policies by replying: "Look at what Hitler did!" This is moral relativism of a very strange sort. Where have our standards gone off to? There are many, many, many people sitting in jail in the United States for conduct that doesn't even begin to approach Saddam Hussein levels of badness. And yet I don't see George W. Bush commuting peoples' death sentences to the words, "hey, it was just murder, not genocide."
When you look long into an abyss, the abyss also looks into you.