For example, I see a lot of praise going out to people such as Genghis Khan, for saving the environment or whatnot (by intentionally killing 20 million people). However, people regularly bash British or other colonial empires even though these empires never did anything to the same level as this.
It's important to see trends in current criticism in the context of criticism that preceded it. In the case of Genghis Khan, his name has long been associated with intense savagery and its only more recently that people have focused on the better sides of his empire. So it's not that people think Khan was lovely; just slightly better than his longstanding reputation.
With the British it's rather the opposite; for a long time we saw ourselves as exponents of order and civilization across the world and it is relatively recently that imperialism has been seen in a harsher light.