The best option would be leaving the Singer alive, healthy, and free, especially if the Singer would be willing to remain and protect this island and all of its inhabitants.

Doing nothing allows the Singer to remain in anguish, and visit even more horrors on everyone here. When all the humans and mermaids are Stricken and gone permanently mad from it, the dominating will still wins.

There is no good option without severe risks. We simply have to find the option that is least disastrous for everyone involved.