Responses
Probably because he doesn't have the ability to set them free. He'd easily get caught and make the situation worse for the mermaid. So he thought it would be better to end their suffering if he can't save them.
beta
But why kill pretty mermaid why not save and let her free. Yo fishman if ya want to kill someone kill that filthy ass man who captured her and those other filthy humans who watched her. Its not mermaid fault so put that kitchen knife away from her no one wants sushi right now