The detective hasn’t even shown up yet and I KNOW I’m being tricked by latching onto this before I’m even into the meat of the story, but I really feel like this is what went down in the past segments… just from vibes-based detective work…
Spoilers! Unless I’m wrong, which I am!
He has by far the cruelest, most narcissistic personality of any of the characters (or so it seems) and he doesn’t even try to conceal it! He’s a well-respected surgeon and extremely handsome and composed so when the evil jumped out during casual conversation it was a bit of shock. Even Mori, who also has that kind of streak, is too scared of compromising his image to agree with him when he starts talking.
I’m still pretty confident my current theory is correct, but I believe Mitamura’s surgical skills may have been involved in the matter. After all, there have been just too many references to his gold ring. Perhaps a couple of severed fingers were swapped around?