Fritz Haber invented a process that synthesises ammonia, which is crucial to fertilizer production. Prior to this process, countries went to war over Guano , one of the previously best available sources of the stuff. Today, the Haber Process feeds approximately half the world's population. Naturally, the guy got the Nobel Prize for this monumental achievement.
Great man, you might think. However, he also lead the team that developed poison gases for use in World War I and went to the front lines to oversee its deployment, for which he ascended to the rank of Captain in the German army. His wife wasn't too impressed by this and killed herself after an argument with Haber. His 13-year-old son found her body. On the same morning, Haber left his grieving son to go the Eastern Front.
Cruel, cruel man, you might think. However, there's another tragic twist to his life. He also oversaw the team that developed a cyanide-based fumigant and insecticide initially known as Zyklon A. Haber's team added a strong "warning smell" to the formulation, so people would be aware of the gas' presence. Some years later, the Nazis took Zyklon A, removied the warning smell and used it for the mass extermination of millions of Jews. Haber was Jewish, and members of his family perished in concentration camps. Haber himself died on a journey to Israel, where he would have become the director of what is now the famous Weizmann Institute.
His inventions feed billions. They also killed millions. I am not sure whether any other single life embodies the moral ambiguity sometimes immanent to scientific progress.