Koroshiya no Eigyou-jutsu

Torii, a genius salesman who consistently ranks at the top of his class in every company he works for, encounters a hitman one day during a client visit. Just as he is about to be killed to silence him, Torii begins to speak— A natural-born devil ravages the underworld of demons and monsters! A rollercoaster mystery that will overturn your expectations begins now!

(Source: Kodansha, translated)