Sexist tropes used in the Harry Potter books: “The main female character will be attacked by a monster, while ze's in a bathroom, and ze will need the male main characters to save zem (despite the fact that ze's generally much more competent than they are).”

Before I started writing Voldemort's Children, I didn't think of the Harry Potter series as being particularly sexist. But now that I've started scrutinizing it more deeply (as I have to, in order to write a good fanfic), I've started seeing the sexism everywhere.

As for the troll itself, I based its behavior on real-life animals. Wild animals usually don't attack humans (or other animals) unless they intrude on their territory or come near their offspring. The only reason an animal would go alone into an unfamiliar place and attack people, is if it was a predator hunting prey. But the troll probably isn't a predator. An animal that is large, slow, and stupid would not make a good predator. I'm guessing that trolls are omnivorous, something like bears.

The other possibility, I suppose, is that trolls were originally created by magic, and were specifically designed to be violent. I think the “trolls aren't monsters, just wild animals” interpretation is nicer, though.