The 22 May Joplin tornado killed 160 people, the most people killed by a single tornado in the United States.
Source: Army Corps of Engineers
The NWS recently issued a report on the tornado, and there is also a nice article on Andrew Revkin's Dot Earth blog. Have a look. Preventing such large losses of life is more than just a meteorological challenge. Societal, economic, and engineering factors are important contributors and need to be addressed for society to become more weather and climate resilient.