My friend Outi from Finland writes:
I am not a specialist on this but I have the impression that Finnish people have developed their idea of mermaid mainly according to H.C. Andersen fairy tale and/or Disney version of it. Anyway, mermaid is not an evil thing if you ask about it from a Finnish person.
We do have a thing called Näkki or Vetehinen (water spirit) amongst our traditional mythical creatures. This is an eveil minded spirit with ability to change its form and its aim is to tempt people to go too deep to the water and drown. There are different kinds of traditional fairy tales
telling how Näkki does his trick (looking like a beautiful female/handsome male, a tame horse pretendeing to be willing to carry the careless to the other side of the water etc.), but anyway the reason for these stories is the same as the evil mermaid stories you heard about: Preventing children to swim too deep and explaining "mystical" dissapearings of people who had gone to for example fishing and never came back.
The difference is that here in Finland we do not believe in Näkki or Vetehinen any more. We just think it is a valuable piece of our tradition and therefore it is thaught amongst other forgotten mythical creatures on history lessons at school.
Nowadays children just have to believe that it is dangerous to swim too deep, because their parents say so. :)