hmmmmmmmmm, I wonder what's the trigger that makes the mermaids violent?

I wonder, too. Could it be uncertainty? Since you aren't hunting them wholeheartedly, then you will be the hunted.

In the case of Asami-chan, she mimicked them: dressing as a mermaid when she wasn't one.

That's true... Then the price was making her one of them for real. I think they have multiple triggers.

If you read the last chapter, it explains why she got attacked- because the guy she liked was in love with her as well

Yeah, I figured after the latest chapter.