Sort of a spoiler but not really a spoiler (the way Dune is written there aren't really spoilers), the series is about saving humanity by creating a leader so bad that they never let themselves be controlled again. In Dune Messiah, Paul (not the one who finally saves humanity because he couldn't be bad enough) compares how many people he's killed to figures in History, like Hitler and Genghis Khan.

He [Ghengis Khan] killed the way I kill, by sending out his legions. There's another emperor I want you to note in passing - a Hitler. He killed more than six million. Pretty good for those days.