As you say, fiction writers have to take responsibility for writing characters that we born in the dark parts of our consciousness. And for choosing to allow those characters to do what THEY would do and not what WE would do.
A character need not be good to be a good character. And even an admirable character--a hero--cannot long sustain a reader's interest without showing less-than-admirable impulses and moments of weakness.
I must take issue with your contention that a fiction writer doesn't have to be careful with factual details. We make up stories, and sometimes (but not always) make up settings, but we can't play fast and loose with biology, history, geography, psychology, physics, and other aspects of the real world. Readers give up quickly on a writer who hasn't done his research.