Yes, I am aware of that. But in classic history this seems to be the same. Good and great is having conquered much. And also in the general perception. The hollywood movies of Caesar or Alexander? Pure admiration. No serious criticism(or any at all), I remember of. Nothing against presenting history as it was. But there is still lots of glorification going on.
My school history classes were also speaking mainly great of the "great" people. Even though we would call them brutal dictators and barbaric enslavers if they would exist today.
"World's [blankiest] [blank]" is generally a pretty popular genre, regardless of the subject. Whether it's the tallest mountain, most venomous snake, longest railroad or fastest plane, people are fascinated with extremes. I think the obsession with the world's 'great' leaders like Genghis Khan or Alexander is just another manifestation of this.
It is, but would you not object to calling "Hitler the great"? Even though he was great by standards of conquering and slaughtering?
The word 'great' is not simply a more emphatic word for 'good', though in some contexts it might mean that. Great Britain isn't great because it's better than the other Britains, but because it encompasses all of the British Isles. I think it may also be the case that people are called great when their impact on history seems to have a great scale. For instance I think Catherine the Great is called great not because she was particularly benevolent (she personally owned half a million people), but because Russia became more powerful and expansive during her reign.