A benevolent dictator, Plato's philosopher-king, can do a great amount of good. Problem is, most dictators are rarely if ever benevolent or wise.
I believe this to be a fallacy. Only I know what is best for me. In order to do good for all and not just some, a benevolent dictator must also be omniscient. I don't believe in the existence of gods, so that idea is right out.
What is good for you, is not necessarily what is good for the collective.