The World Government is the most powerful organization in One Piece. It has created a false image of being a righteous and honorable organization that helps people throughout the world, but that couldn't be further from the truth. The World Government came into existence after twenty countries banded together to take down the Great Kingdom.
Following its success, other nations began to join the organization, and with time, it became the World Government that people know about. It is headed by Imu, who sits on the Empty Throne and the Five Elders are answerable to him. Over the course of One Piece, the World Government has taken many selfish decisions that have caused a great deal of harm to other people.
7 Ohara Massacre
The first thing that comes to mind when someone talks about the evil nature of the World Government is the Ohara Incident. It took place many years before the current timeline, when the World Government received information about the scholars trying to decipher the Poneglyphs. The World Government has made it clear that any kind of action that could harm its sovereignty would entail harsh punishment.
After the organization's suspicions were confirmed, the Five Elders gave the order to kill every single scholar. Hundreds of people were killed only because they wanted to learn about the true history of the world. Nico Robin was the sole survivor of the tragedy, and she was chased by the Marines everywhere she went.
6 Allowing Slavery
For an organization that presents itself as just, the World Government has done many things that directly harm the common people. One of the worst things that it has done is let the Celestial Dragons do whatever they want. The Celestial Dragons have taken full advantage of the fact that they won't be stopped.
This has resulted in them owning slaves, whom they treat worse than animals. These slaves have no respite, and if they ever displease the Celestial Dragons, they are immediately disposed of.
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5 Covering Up Lies
The World Government has been in existence for a very long time, and like any other organization, it has made errors in judgment. But it has managed to keep all the failures hidden from the people. The World Government agreed to the creation of the Shichibukai system, which no doubt had its perks. It allowed the World Government to enlist the help of strong pirates in times of need.
However, these pirates would often take advantage of their status and commit atrocities without being held accountable. Both Crocodile and Doflamingo are excellent examples. In the first case, the World Government managed to cover it up, but the second time, things didn't go according to plan.
4 Destroying The Great Kingdom
The World Government was created with the sole goal of destroying the Great Kingdom, which existed before the Void Century. Going by the limited information, the Great Kingdom's technological advancements made other countries fear them.
So they decided to ally with one another and destroy the kingdom. There is no evidence that supports the claim that the Great Kingdom indulged in evil practices, so it is fair to say that the World Government did it out of spite. The destruction of the Great Kingdom meant all the people living in it were killed and nearly erased from history.
3 Vile Experiments
The World Government's desire to stay in power has made it do many deplorable things. It has tried employing malevolent pirates and creating weapons of mass destruction. The World Government employed Vegapunk, who was forced to create all kinds of weapons.
The World Government gave the green light for various immoral experiments. It happily used the resulting weapons to invade and destroy any country that tried to question its authority. The countries that are unable to pay the Heavenly Tribute receive no help from the World Government and are left to rot.
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2 Killing Hundreds of Children
When the Roger Pirates conquered the Grand Line, they became the biggest threat to the World Government's sovereignty. After Roger turned himself in, the World Government got a whiff of him possibly having fathered a child. Without wasting any time, it sent Marines to all the places that Roger had visited.
Then they began to kill any children they suspected of being Roger's offspring. There was mass infanticide, and it is plausible to assume that hundreds of innocent children were killed. This shows how vile the World Government truly is.
1 Flevance Tragedy
Flevance was a country in North Blue, and it was Trafalgar Law's homeland. It came to be known as the White City because of Amber Lead. The people of Flevance harnessed this resource and used it to create various kinds of products. They had absolutely no idea that Amber Lead was toxic and that overexposure could lead to death.
The royal family of Flevance and the World Government were aware of its toxic nature, but they chose not to divulge the information. Instead, the mining of Amber Lead continued until all the citizens became sick. The royal family immediately escaped from Flevance with the help of the World Government. In order to stop the spread of the disease, the entire country was quarantined. However, the people grew restless and tried to get out, but the neighboring countries killed all of them.