Fritz Haber arguably saved and killed more people than any other single person in history.
Synthetic ammonia vastly lowered the cost of making fertilizer, explosives, and other chemicals.
The process to create synthetic ammonia was a concurrent invention. That is, two scientists came up with it at the same time independently of one another.
Because it allows for inexpensive fertilizer, the Haber-Bosch is responsible for approximately half the food grown in the world today. Fritz Haber, who both invented and also commercialized the process, saved billions of lives.
However, there is a darker history. Haber was a German Jew, a key German chemist developing chemical weapons for Germany in WWI. He oversaw their first use at the Second Battle of Ypres, where approximately 67,000 allied troops were killed in one gassing. His first wife committed suicide after learning how many people he helped kill.
Later, the institute he founded invented Zyklon A. Nazis used a successor chemical, Zyklon B, to murder millions in death camps including many members of Haber’s family. This caused his second wife to leave him, with the marriage ending in divorce.
Both, like Haber, converted from Judaism to Christianity though the Nazis did not care and banned Haber from his lab. He escaped Nazi Germany but died soon after the Nazi’s ascent to power in Basel, Switzerland.
Haber won the 1919 Nobel Prize in Chemistry but died a miserable man.