How do characters with dark pasts like criminals, assassins, and psychopaths increase the suspense of a story?

This should be mostly due to our human tendency to be drawn to danger. Our adrenaline is hungry for some action and couldn't be better fed than through activities of such "dark" characters. Also, almost everyone possesses in their private cupboards, dark characters and alter ego that they might only get to let loose in the pages of such dark-themed plots.