Monkey D. Garp always abided by his personal justice.

SUMMARY

One Piece fan theory explores the reason why Monkey D. Garp never attacked Red Haired Shanks even though he inspired to become a pirate.
On the other hand, Monkey D. Garp attacked Blackbeard to save Koby and pass down the torch to the next generation of marines.
Garp never had any ill feelings towards Shanks, and the only thing that angered him was he influenced Luffy even though Garp wanted him to become a marine.

One Piece has often raised questions in fans’ minds about certain scenarios. Even though there are several fan theories to justify those answers, most of them remain unanswered. One question that has often lurked among the fans is why Luffy’s grandfather, Vice-Admiral Monkey D. Garp, never launched a full-scale attack against the Red Hair Pirates.

No one ever rightly found out the correct answer to that question since Eiichiro Oda never mentioned it. But one of the latest One Piece fan theories delved deep into the matter and found an answer to why Garp never attacked Shanks even though he poisoned Monkey D. Luffy’s mind and influenced him to become a pirate as he grew older.

Monkey D. Garp Never Hated Shanks in One Piece

Monkey D. Garp
Monkey D. Garp always wanted his grandchild to become a celebrated marine, along with Portgas D. Ace. But neither of them followed the path he set for them. Even though Ace had a different backstory, he tried his best to influence Luffy to become a marine.

But when Monkey D. Luffy met Shanks, his will to become a pirate strengthened. For that reason, Garp had always been angry with Shanks but never attacked him.

But during the Egghead Island arc, he launched a full-scale attack on the Blackbeard Pirates to rescue Koby. So, why did he not do the same with Red-Haired Shanks? One of the fan theories claims that Monkey D. Luffy has always had a pirating heart.

Even though his grandfather wanted him to become a marine, he was never ready to go down that path. The only thing Shanks did was to motivate him to follow his dream.

Shanks
On the other hand, as for Blackbeard, he had a reason to save Koby, and more than that he wanted to pass the torch to the next generation. Garp had his personal sense of justice and always abided by that rule. He believes in personal freedom and never stands in front of Luffy’s freedom to do anything he wants. As a marine, he did not want to impose something on anyone.

Moreover, Red Haired Shanks sacrificed his left arm for Luffy. He saved him from the mountain bandits and the sea monster. Garp going against someone who showed such morality would bring dishonor to his badge, and taking on a Yonko like Shanks is something that cannot be done so casually.

Monkey D. Luffy Comes First For Monkey D. Garp

Shanks passed his straw Hat to Luffy
Monkey D. Garp always prioritized his close ones when it came to choosing between duty and safeguarding. He raised Portgas D. Ace as his grandchild even though he was Gol D. Roger’s son. Even though he was present when Ace was being executed, he was full of guilt, and when Akainu killed Ace, he was on his way to face him but was held by Sengoku.

Garp always loved his grandchildren and believed in them. When he heard Luffy challenge Big Mom and Kaido, he laughed because he was confident about Luffy’s victory. The only thing that makes him angry is Shanks. There is hardly any hatred between them since they have known each other for decades, especially since he was on Roger’s crew.
Shanks sacrificed his arm for Luffy
The fan theory explores quite a bit as it attempts to answer why Garp never attacked Shanks. The reasons seem more than enough to justify the question. But still, it is a fan theory, and nothing can be confirmed until Eiichiro Oda himself confirms it.