Abraham Lincoln and Martin Luther King, Jr. were both assassinated in April.
Hitler’s Birthday is also in April.
The American Revolution (Started with Paul Revere's Ride: April 18-19 April 1775)
The Bosnian War began in the first days of April, 1992
The Rwandan Genocide began in April, 1994
The 1906 San Francisco Earthquake (April 18, 1906)
Chernobyl nuclear accident (April 26, 1986)
The 1992 Los Angeles Riots after the Rodney King verdict (April 29, 1992)