I actually thought that Reiko's character development felt unearned, so I'd probably put the turning point before her death, but the decline definitely hastened afterwards. I guess that was largely because she was the closest thing to an overarcing antagonist for that thing, and all they had left were either minor characters with no hype who were unceremoniously killed off, and Gotou who, no matter how powerful he was, was always portrayed as a small scale, personal-level threat to shinichi alone rather than something big or ambitious.
Actually, it's kinda weird how the ending arc almost feels like an anti-thriller -- in lieu of of a standard kind of escalation, that arc was the show at its least personal, least inquisitive, least engaging, least overall threatening by the end, and practically everything ended on an anti-climax, like with the evil parasite conspiracy just being routinely disassembled by a swat team (somehow, despite everyone on said swat team getting killed by gotou immediately after), or gotou dying to bacteria, etc. it's kinda like it really stopped caring by the end and was just looking to wrap itself up.
It also felt like the author just forgot about the serial killer and had to conclude his storyline.
I really didn't like that character. He was entirely out of place, and I didn't think his ability to detect the parasites instantly made much sense. I know he's a psychopath, but really?