Questions one and two can be combined really.

It comes down to morality and sex.

Anita started out being a strong woman who didn't
have casual sex. It is stated over and over again in
the early books. I can see where the series gained fans
because of her moral code. She also was a "monster" killer.
This is/was her claim to fame. She also only killed in extreme
circumstances.

Slowly she was seduced and now is pretty much one of the monsters.
This is enough to bother people because it Scares them.
Could they be seduced by the dark too?
If she had stuck to her morals this would never have happened. That's Scary for some people.

She is able to kill and even torture much easier now and hell the woman has to have sex several times a day in order to feed and keep those tied to her metaphysically healthy. The zealots want to know what happened to their moral hero?

They can't let it go. The character they loved so much has let them down and they can't forgive her for it and they have to do something about it.
So they villify the characters, author and other fans. It has become a crusade for them right up there with religion, politics and abortion.

Viki