That's not the Mearsheimer I know. The Atlantic Monthly piece and the related article in IS are occasional writings. To get what Mearsheimer is really about, you have to read "The Tragedy of Great Power Politics."
There, it's clear that he's not a "pick up the white man's burden" kind of guy. For Mearsheimer, the whole business of "failed states" or "rogue states" is a piece of idiocy. He believes that unipolarity is a passing phenomenon, that the U.S. will have a new great power competitor in the 21st century - China - and that the task of American foreign policy is to prepare for that competition. Pissing around in Iraq is a costly distraction from that task. Moreover, he has no illusions about any supposed moral superiority on the part of the U.S., which is just another amoral great power in his book. So, not surprisingly, Mearsheimer has been a VERY vocal opponent of the war in Iraq.