Well, obviously it's not the most representative example, but yes, if a country intends to kill hundreds of thousands of people, then an atomic bomb is probably the most cost-effective way, even after accounting for R&D. Moreover, if the calculus is how to win the war with the lowest number of additional lives lost, the atomic bombs dropped on Japan were quite likely significantly less deadly, even when comparing just against the expected number of Japanese civilian casualties from the alternative scenario of a Normandy-like invasion of Japan.

EDIT: It's worth saying that humans have been killing each other from the dawn of humanity. Studies on both present-day and historical tribal societies generally show a significantly higher homicide rate than what we're used to seeing in even our most dangerous cities and across our biggest wars.

A bit old, but extensive numbers - https://ourworldindata.org/ethnographic-and-archaeological-e...