April 15, 1912 - Sinking of the Titanic which killed 1,514
April 16, 1947 - Explosions at Texas City killed at least 581
April 4, 1968 - Martin Luther King Jr assassinated/
April 19, 1993 - 76 people die in Waco at the Branch Davidian compound.
April 19, 1995 - Oklahoma City bombings killed 168 people.
April 20, 1999 - Two seniors at Columbine High School shot and killed 12 students and one teacher.
April 16, 2007 - A senior at Virginia Tech shot and killed 32 people and hurt 17 others.
April 15, 2013 - Boston Marathon bombing which killed three people and injured over 250 others.
Edit:
April 2, 2014 - Second Fort Hood shooting
April 17,
April 10, 1979 - Wichita Falls tornadoes killed 54.