Mister Marathon was The Seven's original speedster in The Boys, but the character's fate remains a mystery. The closest the forgotten supe has come to an in-the-flesh appearance on the show has been in storyboards from the fictional in-show Dawn of the Seven movie. Otherwise, he has only been mentioned in conversation by Vought employee Ashley Barrett (Colby Minifee) and fellow former Seven member Lamplighter (Shawn Ashmore). He and Lamplighter got into mischief at Vought Tower together, sneaking in college girls through the back door together, showing that Mister Marathon had the same hedonistic tendencies as the other Supes. His career with The Seven was cut curiously short, however, and he was replaced by A-Train (Jessie Usher), who has lasted four years longer than his predecessor as a member of the team.
Although Mister Marathon has only been brought up in passing so far, there must be more to his story. Being part of The Seven is the ultimate goal of most supes, and leaving the team voluntarily would be improbable, if not downright impossible. This suggests that details of what really happened to Mister Marathon are being swept under the rug and that these details may be grisly.
The Boys comics revealed Mister Marathon's dark fate: the speedster was accidentally killed by Homelander while on a mission. While the details of how exactly it happened are unlikely to be directly adapted for the Amazon show, his killing could be brought into the show's canon. The show could even go on an entirely different, yet equally morbid route.
In Garth Ennis' original The Boys series, The Seven were tasked with saving a plane hijacked by terrorists on 9/11. Homelander and Mister Marathon planned to rescue the passengers, but the attempt was botched by Homelander's ego. His carelessness led to the death of an innocent passenger, and the supe burst the panicked civilians' eardrums with his super-sonic voice. Homelander wanted to abandon the plane entirely, but the speedster begged him to try a last-minute plan. Homelander slingshotted his teammate at the plane in hopes of changing it's trajectory, but Mister Marathon was instead decapitated on impact, and the plane crashed into the Brooklyn Bridge. The Seven's failure, and Mister Marathon's gruesome death, were covered up and he was quickly replaced by A-Train.
While season 1 of The Boys included a failed plane crash rescue already, Homelander causing Mister Marathon's death could still have happened during a different mission. Homelander has injured his fellow Supes on a whim before, and one of these spontaneous attacks could have been Mister Marathon's undoing. The multiple speedsters in the world of The Boys suggest having super speed is not considered that "special" relative to other power sets. Speedsters could even seem somewhat ordinary to Homelander, and he could regard them as replaceable commodities that can be sacrificed with little consequence.
Mister Marathon's fate could even be changed entirely to tie into his replacement's story. A-Train faces pressure from Vought to be the fastest speedster in the world, and abused Compound V to maintain this status. The consequences for not being fast enough would have to be disastrous for A-Train to go to such extreme lengths. It is possible that his predecessor paid the price for being second-best, and was taken out by Vought directly. Regardless of how Mister Marathon met his demise, directly revealing it on The Boys would show how deep Homelander and Vought's depravity runs.