If they actually exist then more than likely both possibilities of mermaids are possible.
They have similarities to us just for being half fish and not merely just animals. So more than likely they can and would be capable of both good and evil, and the choice of either is entirely on them.
Whereas to animals and fish I view them entirely as "neutral" and because of "Survival Of the Fittest" and the "Food Chain" animals NEED to eat and slaughter each other just in order to survive, and only the strongest, quickest, fiercest and most intelligent of them comes out on top and have the best chance of survival. There is really no right or wrong when it comes to that. Or maybe it can be viewed as "Might IS Right".
Both depending on the religion the mermaid is from. All around the world there are different religions that have mermaids in the and they can be either good, bad, or both. I'm torn with this one, but I'll go with good to be safe.
My sister had two of em staying at her house for a few months and they seemed to be alright. The always stayed in her bathtub and every time I stopped I'd go in and take a dump on them. They'd always giggle and take turns guessing what I had for supper.
Traditionally, I think they were evil... luring men at sea into the water with their beauty, and then drowning them, or something like that. Fairy tales and Disney made them more friendly, but their origins were not pretty.