Words have components and can convey different ideas depending on context and usage.
- Great!
- Well that's just great.
- (Asking the wife how she feels about the impending inlaws vising) fucking great.
This is like a recent conversation I had about respecting Putin. Doesn't mean I admire the guy or would even like being near him. It's like a fight analyst saying you gotta respect his hands, or the kind of weary respect towards an angry strange dog: you don't pay attention to the potential it could bite you in the ass. Support vs respect.
Same idea. Complete pieces of shit such as Hitler or Stalin had great impact on the world. As in "fucking great, shit's all fucked because of them". Language is complex and seldom binary. 1 means the singular and nothing else.
That's my take at least.
Edit: scale of impact should be the easiest way to convey the idea. The great cataclysm type of thing.