I can see where you're coming from because it could've been implied rather than completely shown or at least parts of it could've been omitted, but I believe the author's intention wasn't to make the character "grow", the point with this specific arc was to show how cold-blooded Brett is.
Even through an extreme stress situation, he is focused on being rational rather than his own feelings regarding what is happening to him. In the same way that he lacks the ability to empathise with others, he doesn't care about himself THAT much (at least physically) which is why he was also in several occasions putting himself at risk.
My interpretation is that this "proves" that Hayden is so important to him that it caused him to not only FEEL things positive and negative (regret, sadness, urge to protect, worry etc), it made him so irrational that he was actually chasing something when most things were given to him or he'd just take them because he felt entitled to.
Brett isn't the type of person to let emotions or people linger if they lose their purpose which is also why he wasn't trying to get revenge on the rapists. One of the few times he was looking for the score to be set even was when Hayden was involved and wronged.
That scene in specific was to show us this absurd contrast and contradiction in Brett's personality and attitudes. It may not have been the best choice, but I don't think it was just for an obsession with rape in this case.
The overall story was good. But that random ass gangrape? It was so unnecessary. The author ignored it like it didn't even exist. So why draw that in the first place?? These bl authors are obsessed with rape. At first, I thought the rape may have great relevance later in the story but seriously?? None. And the bottom was so NONCHALANT about it. His attitude about it was like "oh,well". Like it's a regular thing for him. And those rapists were seriously leg go of? It was better off as short manhwa. To think I was so excited when new chapters were uploaded. Disgusting.