Star-Lord is the type of hero to throw around jokes in the face of world-ending threats. It's made him one of the most endearing heroes to ever be born from the House of Ideas, but it's also cemented his status as one of the most immature heroes among his peers. While he's tried to present himself as more mature and take on responsibility before, Marvel confirmed he should never fully leave behind his childish persona because it leads to an incredibly tragic future.
Old Man Quill (by Ethan Sacks, Robert Gill and Andres Mossa) introduced a weathered Peter Quill who'd given up his position as the roguish leader of the Guardians of the Galaxy to become the wise ruler of Spartax because he wanted to appear more "grown up." But his decision directly led to the destruction of every single person he cared about, singlehandedly showing that he should remain the man-child he's always been.
The series began with Star-Lord in the middle of launching a surprise attack on the Universal Church of Truth and saying he doesn't keep in contact with any of his former teammates. Just when he thought he had the upper hand, the Church revealed they had launched a surprise attack of their own that destroyed Spartax. Fifty years later, Quill received a video transmission from the remaining Guardians right before their death that told him to find the Ultimate Nullifier to defeat Galactus. Racked by guilt from letting everybody he cared about die, he imagined his dead teammates by his side as he journeyed to Earth.
The Universal Church eventually caught up with him at the Baxter Building before he found the Nullifier. He opened a rift in space-time that swallowed the Baxter Building and the mind-controlled Imperial Guard. Quill entered the rift to find the Nullifier only for it to be destroyed by Doctor Doom as soon as he came back to the present. When Quill was about to be executed, Galactus arrived to consume Earth. Using a sliver of the Time Stone he lifted from the time machine he got in the past, Star-Lord shot Galactus in the head, effectively defeating the Universal Church.
One of the main elements of the comic is Star-Lord being haunted by his guilt from abandoning the people he cared about for the "grown up" life he thought he wanted. Essentially, everything that goes wrong for Quill stems from him running away from the person he truly is and attempting to adopt a "mature" persona as ruler of Spartax. That need to become an adult and cede his childish sensibilities leads him to leave the Guardians, a team he loved with all his heart. While the story presents him as being happy with his decision, it slowly unravels that he always felt a part of him was missing when he became the ruler of an empire. Quill constantly says that choosing Spartax over the Guardians was the right thing to do, but the only time his plans go the right way is when he stepped back into the persona of the gunslinging overgrown child he used to be.
Ultimately, Quill's adventure establishs that Star-Lord should never try become the mature emperor many expect him to become. Instead, he should remain the wise-racking adventurer he usually is throughout the comics and the MCU. The comic cements this idea by showing him creating a new Guardians of the Galaxy at the end and it's the only time he seems happy since his family was killed. While Star-Lord's lack of maturity has led to a lot of problems for him, it's clear that worse things can happen if he decides to "grow up."
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