They say you either die a hero or live long enough to become the villain. Unfortunately for these 29 people, they kept their skeletons in the closet instead of in the ground and paid the reputational price. 

Fritz Haber was a German chemist who rose to prominence in the early 1900s. He was instrumental in transforming farming to the modern age, specifically when it came to fertilizer. In 1918, Haber was awarded the Nobel Prize for his "Haber–Bosch process," which created ammonia from nitrogen and oxygen gasses right out of the atmosphere, facilitating the mass production of fertilizer. However, once World War I moved into full swing, that same research turned into something much more cynical. By the end of his life, he was known as the "father of chemical warfare," and was the mastermind behind mustard, chlorine, and phosgene gasses. If only he had just stuck to farming. 

World War I was also responsible for earning France's Philippe Pétain a reputation, but for the heroic general, it was a good one. That would all change a few decades later, however, when he became the head of the collaborationist regime of Vichy France, in essence helping Germany occupy his own country. Had he passed in his early 80s instead of his mid 90s, that could all have been avoided.