Every show has a character that is loathed by the fandom. Something about the character's personality or actions gets fans worked up to the point that every forum is full of threads complaining about the character. And there is no One Piece character more hated than Sakazuki. Browse any One Piece conversation, and someone complaining about him will quickly turn up.
Sakazuki, also known as Admiral Akainu, is the fleet admiral of the Marines, the World Government's military sea force. This group is entrusted with enforcing the laws of the sea and bringing rulebreakers to justice. Sakazuki acquired this title by defeating Aokiji after the previous fleet admiral, Sengoku, retired.
Sakazuki is hated by fans because he killed fan-favorite character Portgas D. Ace, Luffy's older brother. When Ace was captured and sentenced to death, the Summit War of Marineford happened. This war saw the Marines and several pirate crews clash as the pirates tried to rescue Ace. However, when Luffy and Ace's relationship was revealed, Sakazuki attempted to kill Luffy. However, Ace got in the way, taking a fatal blow for his little brother. This was an emotional moment that made most readers see Sakazuki as a horrible person.
Another reason Sakazuki is hated is because of his dedication to Absolute Justice. This philosophy is at the core of the Marine doctrine and states that stopping all illegal acts and lawbreaking is worth it, no matter the human costs incurred. Absolute Justice also extends to those who interact with or are related to lawbreakers in any capacity. No matter how vague, in the eyes of the Marines, the relation is just as evil as the criminal -- even if they have done nothing wrong and broken no law. This is mixed with a starkly black and white view of the law, where the situation surrounding the illegal activities is not considered. Meaning that someone who accidentally breaks a minor law is considered just as evil as someone who has deliberately broken thousands of laws.
Sakazuki takes this already warped philosophy to the extreme, especially when it comes to pirates. Sakazuki is willing to endanger hundreds of people as long as a pirate is killed or captured in the process. This has led to him committing horrible acts, including destroying a ship full of refugees, on the off chance that one of them was an Oharan scholar. He's also executed his own men on several occasions, especially when they display what he considers to be cowardice.
This makes Sakazuki easy to hate because he acts as a perfect metaphor for how a justice system can perpetuate injustice by becoming too fixated on the law as a rigid concept rather than a human one. By becoming too consumed by the dogma, Sakazuki has ended up with totally warped morals. In fact, people are more likely to be harmed by Sakazuki and the Marines than they are injured by pirates, most of whom keep their quarrels between themselves.
However, that doesn't mean that Sakazuki is beyond redemption. In fact, many fans hope that the Marine fleet admiral will realize the flaws of both Absolute Justice and the Marines and try to make amends for what he has done in the past. This could be by apologizing to Luffy for the death of his brother or by helping the Marines move to a less hardline and more human-focused philosophy. But for that to happen, Sakazuki will need a massive culture shock.
Sakazuki is a fantastic villain, and he acts as a great counterpoint to several other characters in the One Piece universe. Despite the over-the-top nature of the series, Sakazuki is a nuanced villain, as, like all the best antagonists, he believes what he is doing is right. However, as readers looking from a neutral position, we can see things that Sakazuki is blind to and thus see the flaw in the dogma of Absolute Justice.
