Death. No matter how you look at it, death is one of the major themes of One Piece. But aside from using it as a way to pick off annoying or unnecessary characters, Oda uses death as a major plot device and symbol to aid the story.
The first use of death in the One Piece storyline is to mark major turning points in the plot or to begin new stages of the story. The first example of this was the death of Gol D. Roger, which kicked off the story in the first place and began the Great Age of Pirates. The deaths of Whitebeard and Ace ended the old Great Age of Pirates, which had been ruled by remnants of Roger's generation, and began the New Great Age of Pirates, which is being revolutionized by the Worst Generation, including the Eleven Supernovas and Blackbeard. On a less grand scale, the death of Sabo was a time of personal change for Luffy, where he stopped whining and grew up a little in his desire to become strong to protect people. One may also argue that the "death" of the Going Merry was a time when the Straw Hats finally left behind the remnants of their past lives and became fully focused on their dreams and the ocean (however, one could also say that this happened during the timeskip, so it resulted from Ace's death).
Another aspect of death that has been used at length is symbolism. Some characters in the series, the World Government, understood symbolism, and knew that Roger was the figurehead of the Great Pirate Era. By killing him, they hoped to inflict a psychological blow upon pirates by symbolically killing the Great Era of Pirates. Roger had his own plans, however, and flipped the World Government's message to say "go out adventuring and become rich", hence starting an era. Perhaps the profound symbolism seen so far in the series is from the deaths of Whitebeard and Ace, which had many meanings. Once again from the point of the World Government, they hoped to send out the same message they wanted to give when they killed Roger, but they once again failed, as Whitebeard restarted hope in the discovery of One Piece (random metaphor: think of when a fire has burned down to the ashes, then you blow on it and add more firewood, making it blaze again). The fact that Whitebeard (as a powerful figure in Roger's era and as the ruler of the seas after his death) and Ace (the son of the Pirate King and a soldier of Whitebeard) represented the old Great Age of Pirates also carries multiple meanings. The death of Whitebeard at the hands of an up-and-coming pirate indicates that the new generation of pirates is overcoming the old (the presence of famous criminals on Blackbeard's crew could also indicate the lawlessness of the New Era, but that's a separate matter), and the death of Ace at the hands of a Marine admiral shows that no matter how powerful pirates may get, the Marines still have a say in the way things are done in the world. Overall, the deaths of these two great figures in the world indicates that the Old Pirate Era has collapsed while simultaneously kicking off the New Pirate Era, which is symbolized by a strong Marine presence, powerful new pirates from the Worst Generation, and a streak of lawlessness, as seen especially in the New World.
A message specifically associated with the death of the two heroes of Fishman Island, Fisher Tiger and Queen Otohime is one of strength, though in two different forms. The death of Fisher Tiger symbolized to the reader a message of strength and defiance, even after his death. By refusing to accept the human blood transfusion, Fisher Tiger showed that he was never going to bow to the humans and that he would continue fighting forever. To the inhabitants of Fishman Island, who saw the incident as one where the humans ruthlessly killed a national hero, this death symbolized the human hatred towards fishmen and the lengths at which they would go to persecute the fishmen. As for Otohime, her death (which was first believed to have been caused by a human) initially symbolized the same thing as Fisher Tiger’s death: human hatred and racial persecution. However, after it was revealed that her death was caused by a citizen of Fishman Island, her death only served to show the depth of Hody Jones’ determination and rage at the humans and his desire to conquer them. There are several more miscellaneous uses of death symbolism throughout the series, likethe death of Sabo symbolizing the death of dreams, freedom, and innocence at the hands of the World Nobles, the passing away of Jaguar D. Saul indicated the lack of patience and tolerance the World Government has for morals in the Navy, and the death of Nico Olvia and the scholars of Ohara symbolized the death of learning.
Aside its use of a plot device, death also has profound effects on characters in the story, serving as character development or as an incentive for character development. Just as Roger’s death sparked the Pirate Era, Sabo and Ace’s deaths began a new period of character development for Luffy, as well as his entire crew. Due to the death of his beloved older brothers, Luffy began to realize the need to grow up and to become stronger, each time leading to a period of intense training. Likewise, the assorted deaths of Kuina, Banchina (Usopp’s mother), Bell-mere, Hiluluk, Nico Olvia, Tom, and the Rumbar Pirates caused the individual character developments of all the characters. Zoro and Chopper followed a path similar to Luffy’s, Usopp began lying, Nami became money-happy, Robin became a sad, lonely adult, Franky became more spontaneous, and Brook became rather reserved and a little eccentric. The death of Fisher Tiger on Fishman Island sparked the hate that eventually gave birth to the New Fishman Pirates and the anger of Hody and his crew. The way a character dies is also an element of character development for himself, such as Whitebeard and Hiluluk’s methods of death expanding upon their defiance and strength of will and character. Bell-mere and Ace’s deaths show the depth of their love for their family and the lengths at which they will go to protect them.
Death has many uses as a plot tool, from being an instrument of character development to a plot marker to a symbol in the story. In One Piece, whenever an important character dies, it serves not only the purpose of getting rid of a character but also developing the story, making sure no death is a waste. One of the things that One Piece is excellent at portraying is that death can be just as important as life in its effects on more than just the dead person's life.
Thank you for reading, it has been an honor to participate in this tournament. Good night.