What one event killed the most people?

That depends on what you mean by 'event' and 'life'. The greatest loss of life on earth was the Permian-Triassic extinction event, which occured 252 million years ago. The greatest loss of human life in a single event could be defined with World War II, the death toll of which, at its highest estimate, is roughly 72 million dead. But the greatest loss of life in a single one time event is the atomic bombing of Hiroshima, Japan by the USAF in August 1945 which resulted in the deaths of roughly 150,000 people in a single day, with many more dying of radiation poisoning and injuries in the days, months, and years to come.