Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei Suru

[DarthEnderX]

This is similar to the worn-out trope of a fantasy world missing exceedingly obvious everyday things, like soap, before the mighty Japanese man teaches the hairy barbarians something that should have been invented a thousand years earlier.

How exactly is the fact that if you wet this very specific type of rock, rub it all over your body, and it'll make you smell different and get sick less...in ANY way obvious?

I was a bit surprised he knows his Mendeleiev table so well, down to several decimals.
He also has a good understanding of matter and metals atomic structures.
I guess he's at least College graduate or even higher. For a tool, he had quite the education and gift for learning.
I would not be surprised he had quantum physics courses.

I'm betting it's a Hitman situation. He has to know everything and be able to do everything, so that he can be anyone to kill anyone.

He already said he mastered cooking so he could pose as a chef to kill someone. He probably learned all that science to pose as a professor for a hit.

Observing the 40 years earlier scene, I wonder if it was still about killing the same hero. Was the goddess originally granting the hero killer 40 years of time to get strong enough to slay the hero? Only when the earlier reincarnation turned out to be a waste of skin, she instead chose the MC, but there were only 18 years of time left for him.

She called the NEET "number two-hundred"-and-something. She is CLEARLY not putting all her eggs in one basket.

My prediction? The Hero is an otherworlder the goddess herself empowered to kill the Demon King, but she made him too powerful and he's a threat to her and she's trying to clean up hermess.