The lighthouse keeper was going further insane. For he was always a little, and then a bit more, after listening to all the unbearably moony songs all by himself. And having succumbed to a case of missing someone he could not bear at all, but who suddenly seemed bearable from a distance, he though up a bad deal.
The lighthouse keeper offered to trade, an ability to swim for him and an ability to walk for the mermen. But he swam not far at all, before the mermen discovered that the shoes have no magic in them and severely need feet to be useful. They frothed up a storm to wash the keeper back to the lighthouse, and considered their next not-steps.
The mermen considered human murder to be nearly murder. And so it was decided to outsource it, for the sake of deniability in the history books and migratory turtle songs. Because turtles are gossipy.
The mermen got a spider. To do it. It was a famed and frightful spider, so mercenary that it always traveled with his own headstone, just in case.
Two things might have happened.
The Moon got so much second-hand embarrassment from all the moony songs, it hid. The lighthouse keeper gave up to the darkness and his bones are still kept by the lighthouse. The spider is waiting in ambush and in vain for the keeper, re-reading Cinderella in its tactics preparations. The mermen are all together in a bar, to make sure they all have an alibi. They are getting old, and tired of each other.
The Moon is the Moon, ask your astronomer or even meteorologist about its entirely predictable sightings. The keeper caught eleven migratory tortoises to take him away. The spider gave up its mercenary ways and enjoys the life on the moth farm, reading up on the Cinderella at its leisure. The mermen are all together in a bar, they have gotten quite fond of the selection there.