It seems pretty obvious to me that this whole sequence isn’t supposed to be taken as literal, I am a little confused as to why people are taking it as literal. Whether this is Sei having delusions, hallucinations, or gaslighting himself, it doesn’t seem like these events are actually happening...
This is just Death Note with, overall, worse writing. That being said, Death Note was amazingly written, so that’s a high bar to try and pass anyway. Though, I am finding it hard to actually like any of the characters.