I've been considering this issue ever since it came up on R.A.A.F.
Argument by analogy is a dangerous business of course.  We aren't 
"guests" in the original author's work, more like burglars.  We can
hardly regulate the actions of fanfic writers since we are basicly 
scofflaws of a minor sort.  

That being said, I'm always puzzled and repelled by fanfic written for 
no particular purpose except to kill and corrupt the heros of the 
original series.  If you like the series, why destroy it, even on an 
imaginary level?  If you don't, why are you writing about it?  I suppose
it makes sense if what you liked about the series were the villains.