This really is a hard question to answer. I really belive the answer is both yes and no. Not everyone is created equal and unforturnatley some do have weak self control.
I will never forget a couple of years ago I was having a disscussion about Fantasy writing with a Christain lady that I think of the world of. She is very down to earth and not very judgemental at all. She was telling me that her Grand Daughter was a avid fan of "Anne McCfrrey". The Grandmother had even read a few of her books and loved them. The problem was, the Grand Daugther actually belived that "Pern" was a real place. The Grand Daughther was 20 years old at that time.
Here then is where the problem lies, some people belive that what ever they read is true. It does not matter weather it is fiction or non-fiction (and belive me, I do not think that all non-fiction is all that factual).
However, I do not think that a author should be held accountable for a fictional story of any type. I think that 98% of all people that read them realize that they are just stories.
Stories for enjoyment
Stories to make you think
Now as to being an author of these stories. You can not blame yourself if someone does something bad after reading one of your stories. Now I will qualify that a bit, if you have a reader respond to you saying that he was going to go out and do some really bad shit, and you repled telling him to go ahead and do it. Well then, I do belive that you done a wrong in that case.
We all have fantasies and they are very diverse, and there is nothing wrong with that as long as they remain fantasies, or are totally between consenting parteners.
But as to my feelings on the question, Yes there is always that small percentage that will belive whatever they read and perform there actions based on that.