Garp let Ace die in front of his eyes, but his only other option was to side with the very pirates who are symbols of anarchy.

SUMMARY

Garp let Ace die because he chose the World Government’s controlled evils over pirates.
If the pirates rule the world then the anarchy would be unbearable.
So people like Garp choose the World Nobles and their atrocities over pirates like Blackbeard.

The dichotomy of One Piece can make anyone balk. On the one hand, you have the World Government, fraught with Celestial Dragons, the most corrupt and ruthless people alive. And on the other hand, you have pirates, some of whom are trustworthy, but the others are no less evil than the nobles.

So, who do you choose as the rulers of the world? When no side has the correct answer, you simply have to choose the lesser of the two evils. This explains why even a corrupt body like the World Government has some morally upright people working for it: Garp, Sengoku, Kuzan, and Kuma are some examples.

But who draws the line between bearable and unbearable evil? How could Garp let Ace die in front of his eyes by the very government he serves? According to a theory, he, too, was forced to choose the lesser of the two evils.

Why Didn’t Garp Save Ace in One Piece?

Garp grieving over Ace’s execution in One Piece | Crunchyroll
Garp adopted Ace after Roger entrusted him to the Vice Admiral. He took the gangly boy in and raised him as his own. Yet, when the World Government decided upon the boy’s execution, Garp never betrayed his uniform and let the boy die.

Most Marine Admirals and other World Government officials, including Garp, have a strong sense of loyalty, honor, and duty. Most of them also know that the World Nobles are the stinkiest pieces of scum on Earth. Yet, they choose to serve the World Government because they, too, have chosen the lesser of the two evils.

According to a theory, their options are limited. Because what else can they do? Side with the pirates? While knowing full well that people like Kaido, Crocodile, Blackbeard, and worse exist? A pirate rule would mean a lawless world. Sure, the protagonists of One Piece are nice, and so are some other crews. But can you say the same about all pirates?

Ace’s death in One Piece | Crunchyroll
The fan theory suggests that Garp knew that they would have to make a decision at Marineford. In fact, if Grap had tried anything to save Ace or Luffy, he would have instantly declared himself an enemy of the World Government. That would result in putting a target on his head and Garp would not be able to continue with the minor transgressions, like sparing Luffy at times.

If someone of Garp’s stature rebelled, it would put the entire world in shock and make them question the World Government. This could lead to a potential imbalance in power. And there’s no telling how evil pirates would spin the situation in their favor.

The World Government is a Necessary Evil

World Government serving the World Nobles in One Piece | Crunchyroll
If you think that the Marines, Admirals, Vice-Admirals, and other employees of the World Government don’t know the evils they commit, you would be dead wrong. Anyone with two eyes and a brain would know how evil the World Nobles are. But they are simply obeying their demands for the sake of the greater good.

Sure, the Celestial Dragons are abominable slimy fungi, but they symbolize justice and order (ironic). Without them, the system would collapse and even prevent the good World Government members from doing their duty. Thus, everyone is simply letting them commit the occasional atrocities for the greater good.

If you think that the pirates are an option then think again. How many good and morally upright pirate crews have you encountered so far? The answer will be a handful; because most of them are selfish, greedy, and power-hungry. They are no less evil than the World Government and are probably worse.
World Noble Charlos getting beaten up in One Piece | Crunchyroll
Eiichiro Oda has modeled One Piece to resemble the atrocities of the real world. And if you know the real world, you know how a perfect answer to these atrocities never exists.