Why kill them? You already have built an NPC - they can leave the party and contribute to the plot in other ways.

Alternatively let them be the first to open the treasure chest and eat up the ancient pirate curse - not quite a heroic death but the other PCs can be glad it wasn't them that did it.

Or do both - the NPC gets turned into a squid and the party needs to find the Misty Witch of the Seven Tides to reverse the curse and turn them back.