Carl William Scheele is one of the most important scientists in human history, because of the sheer scale of his impact.
An incredibly prolific researcher, Scheele discovered oxygen, citric acid (also known as vitamin C), and chlorine gas. He developed a process for refining phosphorus that made his native Sweden into the world's leading supplier of matches. Like Prometheus with flame, his discoveries have paved the way for much of our modern world.
But Scheele also discovered something that caused untold pain, suffering and death for thousands of people for over a hundred years: