Ch. 145 'Dark Force'
Everything in this world always had two contrasting sides. If there is heaven, then there is earth. If one has a light, then there must be dark. It is kind, and it is also evil.
Likewise, humans and spirit as some of them were nice, and some were evil. Humans with evil hearts could change and be kind and vice versa. But it was different with the spirits. Once they become evil spirits, it will be hard to return to normal.
Weak spirit races such as fairies or vlinder[1] will easily turn into evil spirits when their hearts become weak. But spirit races like mermaids were quite strong that it was impossibly become evil spirits even if their hearts are vulnerable or feel insecure.
Merylin could not understand how her mermaid-family could become an evil mermaid. Not just one, but all at once without exception, has become an evil mermaid. It really does not make sense.