American history is replete with men that have "accumulated" vast wealth one way or another, and then their interest turns to attaining power over the masses. It is like an elixir to such people, they are assured that they indeed know what is best for mankind, even if the gun and noose has to be used for mans own good.
Only crack pots and semi-cracked pots come to mind in regards to rich guys wanting to rule the world...can't think of any others. In any event, Gates probably just wants to sell software in China. This is not as benign as it sounds, but neither is it as sinister as could be imagined.