@LikeASkurn,
I respectfully disagree,
Becoming a sadist in these situations is quite unrealistic. People get betrayed all the time in some form of fashion. Of course in these revenge stories they turn up the betrayal to over 100 and always have the MC come back and leave most of his betrayers, especially the female ones, alive for sadistic purposes, even if they become a love interest later.
(Looking at you, Kaifuku Jutsushi no Yarinaosh, etc.) I don't think people would do that. I get that to him the betrayal seemed to happen immediately and then he was returned to the past. He was still in a moment of raw emotion so he acted on it without thought, if we ignore the introspection in chapter one which I suspect is supposed to mean he went insane. Overall, my point is everyone will not resort to sadism or complete rage. People are dynamic creatures, they respond to things differently, although we all have the same emotions and those responses will bear some similarities. Most people will have some moral principles they fall back on that shaped who they are. The betrayal will have to be so mind breaking that it opens up a path for them to turn their back on who they were before it just to become sadistic and pyscopathic. Meaning something in their mind, their very being, their inner core broke and went wrong. I'm not saying that can't happen, but when every Revenge manga almost does the same thing then....yeah. Point being, the average person is not sadistic, sociopathic, or psychopathic. The latter two being mental disorders. That's what I'm trying to convey to you here, so it's absolutely not realistic that in almost every revenge manga, the MC turns into just as much a villain
(sociopaths and pyschopaths aren't always villain-like) as the people they're supposed to be fighting. They become far more sadistic, half the time, than their enemies, even with the author tries to go out of their way to make the MC's enemies seem like over the top vile people.
Stories can make sense. It's the fact we have so many that don't, and that becomes our standard that we believe that while creating stories and thinking through their plots, character motivations, organic reasons for character actions, character emotions, overall world building, etc. won't allow for an entertaining story. Stuff like this works because people like it, no offense to them, just like Isekais develop tropes and formulas because they sell and people indulge in it until they get tired of the same thing over and over again. Authors and creators who produce in a sort of corporate environment and need to eat, have a place to sleep, and aren't willing to test the waters will rinse and repeat what works and sells. That's all. This is the same for any genre, American Superhero genre had formulas until someone came along and challenged it and changed it up and then that became the new formula.
Sorry to be so long winded here, but I don't think it would've been a problem to just have the MC finish her off. She only survived because of forced plot reasons and the justification for it seems to be that MC has gone crazy and therefore lost himself and his previous morals and has opened himself up to doing whatever. Of course, I haven't read the rest of the chapters to prove this point, but I've read enough Revenge manga, well the first few chapters, to know that's the standard formula for these stories. My opinion is people gravitate to them because they see it as something different from standard formulas in other genres and so here we are.
Edited: Actually I have read most of the chapters in this manga because it was often recommended as one of the premiere Revenge mangas over others. Refreshing my memory with a cursory skim, yeah, he turns into pretty much a villain and dropped whatever moral principles he had prior to his betrayal like I said. All in that one instance of betrayal he suddenly becomes a sadist. There's no internal conflict with his previous moral makeup over his new moral guidelines. None. He's kind of turned into a psychopath overnight. I argue that's entirely unrealistic, the average pysche, average mind, doesn't work that way and no, I'm not a psychologist or psychiatrist, just have years of experience interacting with people. Enough to realize people are far more dynamic than character genre checkboxes allow them to be. The world is full of traumatic events, people go through crazy things all the time and most bounce back while others don't and have a variety of responses.
Overall, don't get me wrong. I don't hate a good revenge story. When done right the audience can definitely get behind the MC and embrace their quest to bring some comeuppance, but Revenge Mangas, J-Novels, etc......eeehh.😐