Genghis Khan was a "great" man.
However, he was also very cruel.
Alexander the Great was a "great" man.
Yet, he could launch a siege of Tyre because he felt insulted.
Elon Musk is great, because he has revolutionized both cars and space launches.
He made electric cars cool!
This will have massive impact on how cars are made in the future and on how we fight global warming.
He revolutionized how the US launches rockets. SpaceX has driven the price of launches down.
He may not be a "good" man. He may in fact be a horrible human being.
But, I think he is as "great" a man as any in our generation.