Review Detail of killergut in Death, Devotion, Dissonance

I enjoyed the story until around chapter 200. Where the story is essentially about a kid who has a reincarnated soul in his head. They live with each other, fight, make up, etc. The voice in his head teaches him things and casts some spells for him. Either way, the focus is essentially on the kid and his life, while the soul is really pretty much a side character all that time. But around chapter 200 the soul is given the option to kill the kid and take over his body. Which he ends up doing, so what we were led to believe was the supposed MC just gets killed off like that and the soul is now taking over his body and gonna cut ties with people and live differently. I get that the synopsis and first chapter made it seem lile the guy getting reincarnated would be the MC, but he got turned into a soul instead and pretty much made into what I assumed was a permanent side character position. Either way, I'm pretty disappointed with this turn of events, it feels like all that story and character development the kid went through was just thrown away after all that time, and now we get the side character who had like 80% less development as the new MC.