L took a bit to start suspecting the supernatural, and he used a variant canary trap to narrow down the killer's location, which at a minimum pointed to whoever was making decisions.

Hmm... this high school kid is so unassuming and mind-mannered. That just makes me even more suspicious of him!

• L

They all conveniently died by heart attacks though. If disease is ruled out, shadow demon magic is honestly as reasonable as anything.

IIRC prisoners died worldwide of heart attacks in the span of a few hours. The cops do mention the possibility of it being a coordinated attack from a global network somehow, I think one even jokes they suspect it was the CIA.

But yeah, as to how L starts to think it was supernatural they never really go into it besides "he's the world's best (and 2nd and 3rd best) detective".

A Simpsons treehouse of horrors special.

An actual episode done by an actual anime studio. It's one of my favorite Simpsons episodes ever; the Westworld parody in the same episode is also extremely good.