Question

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

Answer 1

When authors set the main characters as people of questionable backgrounds, readers know that something is always bound to happen because either the past always comes back to haunt them or sooner or later the character(s) might fly off the deep end. So, it's the anticipation of what might come that increases the suspense of a story.