The world of One Piece is massive and, over the years, has seen countless pirates and powerful Navy members take to the sea in an attempt to take the era by its throat. The Pirates aim to get to the place where Roger left the grand treasure of the One Piece and the Marines attempt to stop that from happening.
As with all major organizations in the story, there is always some sort of discord within the ranks, and often, it leads to betrayals. Sometimes, this betrayal is even orchestrated by the enemy side and at others, it is done by someone on a whim.
6 Crocodile Betrays Alabasta
One of the earliest betrayals seen in One Piece came during the Alabasta arc when Sir Crocodile revealed his plans to trick the entire country. He posed as a messiah for the country against all the invading pirates but in reality, was the one pulling the strings all along from behind the scenes.
Thanks to Vivi, it was made known that Crocodile was actually attempting to run the country into the dirt and so, the princess fought against him along with the Straw Hat Pirates. Crocodile betrayed countless people and even attempted to kill his very own partner, Miss All Sunday, who fans know better as Nico Robin.
5 Pudding Betrays The Straw Hat Pirates
Pudding is the 35th daughter of Charlotte Linlin and one of the key characters to be introduced during the Whole Cake Island arc. She was first introduced after the events of the Fishman Island arc and later, she was revealed to the bride-to-be of Sanji. In the Whole Cake Island arc, Pudding tried to help the Straw Hat Pirates, however, she was actually leading them into a trap.
She betrayed not just the Straw Hat Pirates but Sanji as well and was planning on killing him and his entire family. She was, however, moved to tears by Sanji's kindness and had a great change of heart. Pudding would then go on to help the crew escape from Whole Cake Island by laying her life on the line for them. Her character went through great changes and she is now one of the most beloved characters in One Piece.
4 CP9 Betray The Galley-La Company
Perhaps the most shocking betrayal in One Piece comes during the Water 7 saga when the Cipher Pol revealed their true nature to the Straw Hat Pirates and the Galley-La Company. They posed as shipwrights and worked with Iceburg for quite a while, just to get their hands on the Ancient Weapon Pluton's blueprints. To do that, they befriended the entirety of the Galley-La people and got incredibly close to Iceburg.
In the end, they revealed their true nature and struck when the moment was right. Thankfully for Iceburg, Luffy and the Straw Hat Pirates were on the island and dealt with all the trouble to save Robin and, by extension, prevent the blueprints for the Ancient Weapon Pluton from falling into the hands of the CP9.
3 Big Mom Betrays The Vinsmoke Family
Big Mom is one of the Yonko of the One Piece world and an incredibly powerful pirate. At one point or another, she was bound to come into play in major fashion after the timeskip and that happened during the Whole Cake Island arc, where Luffy and a part of his crew infiltrated Totland in an attempt to get Sanji back and steal a rubbing of the Poneglyphs that Big Mom had in her possession. There, fans learn about the true nature of Charlotte Linlin.
It was revealed that she was just as cunning as she was powerful. She tricked the Vinsmokes into forming an alliance via marriage and eventually planned to massacre them all. Her goal was to get her hands on the scientific technology of the Germa, especially cloning. Thanks to the efforts of Luffy and Sanji, this massacre was thwarted in the end.
2 Kanjuro Betrays The Red Scabbards
Kanjuro was introduced to the fans in the Dressrosa arc of One Piece as an ally of Kin'emon and a samurai of Wano Country. He helped the Straw Hat Pirates save Dressrosa and, later, he played a big role in the Wano Country arc as well. Kanjuro is known to have been taken in by Oden at a very young age and trained to be made into a perfect Samurai.
He served as a vassal of Oden, however, his true allegiance was to the Kurozumi Clan of Wano, being a member of it himself. Kanjuro had been betraying the Scabbards from the very beginning and leaked a lot of information about them. He attempted to thwart the raid on Onigashima on more than one occasion and eventually, revealed his treasonous nature to the group during the day of the Raid.
1 Blackbeard Betrays Whitebeard
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Blackbeard, formerly known by his original name of Marshall D. Teach, was a member of the Whitebeard Pirates once. He joined the crew at a very young age and was taken care of by Newgate himself. Growing up, he admired Whitebeard for his strength, however, that didn't stop him from betraying his very own father.
When Thatch, the Fourth Division Commander of the crew, came across the Yami Yami no Mi, Blackbeard didn't hesitate to murder him in cold blood and jumped ship. He then established his own crew and even made things worse by turning Ace in to the Navy. During the Marineford arc, he killed Whitebeard himself.