Warning! Spoilers for The Boys comic book and television series below!
The horrific death of Hughie's girlfriend Robin is the driving catalyst for the character ing The Boys, and in the comics, he would get brutal revenge on A-Train for the murder. Towards the end of the series, a Compound V-powered Hughie would brutally kill The Seven's speedster, who killed his partner without much remorse, in one of his darkest moments for the hero.
In The Boys comic book and television series, Hughie's life was forever changed after the Supe A-Train traveled faster than the speed of sound and accidentally killed his girlfriend, Robin. The incident would lead to The Boys. While he would eventually develop a relationship with the hero Starlight, Hughie never forgot about Robin or getting revenge on A-Train, leading to one of the bloodiest moments for the character, as he killed the speedster in an absolutely brutal fashion.
In The Boys #63 by Garth Ennis, Russ Braun, Tony Avina, Simon Bowland, and Darick Robertson from Dynamite Entertainment, Hughie meets up with Billy Butcher on a rooftop and discovers he's managed to subdue and chain A-Train to a wall. Butcher plays a tape recording of A-Train talking to Homelander about the incident where he accidentally killed Hughie's ex-girlfriend, Robin. The more the tape plays, the more frustrated Hughie gets, as Homelander and A-Train joke about the murder, saying the speedster looks like a "virgin" compared to what Black Noir has done. An emotional Hughie then loses control when A-Train starts talking on the recording about Starlight and making some disgusting comments about recruiting her to the team for his own pleasure. Hughie curses out A-Train before kicking him so hard his head detaches from his body.
Hughie is the least-violent member of The Boys, so to see him lose control and get revenge on A-Train by brutally murdering him shows his breaking point. Killing Robin and then bragging about what he would do with Starlight pushes Hughie over the edge, resulting in him murdering A-Train. It's possible but unlikely that Hughie will get revenge on the speedster in The Boys television series the same way he did in the comics considering A-Train actually apologized for the incident and (eventually) showed remorse.
Seeing Hughie kill someone out of frustration and anger showed that even The Boys' most selfless, non-violent member was capable of losing control. Hughie didn't have the same murderous appetite as Butcher or other team , with his first kill being a total accident. In the end, Hughie got dark revenge on A-Train by murdering the Supe in the incredibly bloody scene that showed that, ultimately, every member of The Boys, including their "weakest" member, was capable of losing control and killing a Supe.
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