The trolls of Scandinavian folklore is not entirely evil. They are more like a hidden people living apart from the humans but with a understandable society. The same with the huldre people.

The tales differs but in general they want their children to be brought up by humans, and in return they adopt the human child. I checked my books and in some tales the human child dies while living with the trolls and then money is paid to the human society until the Skifting is dead.

Trolls are mentioned in the old pagan sources, together with Thor, Freja and the Joutum - so the idea that whole societies of weird creatures existed are definitely very old. Scandinavia has a lot of pagan leftovers, that were wiped out in the puritan Britain, for example the Nisse. We still put up rice porridge for him in the Christmas.