Here's a top of the head list of the most important assassinations of political leaders in America since WWII, plus attempts on the life of Presidents and Presidential candidates, with a rough grouping of the assassins in terms of left, right, or apolitical crazy.
Harry Truman - Left — Puerto Rican terrorists - Left
John F. Kennedy - Left — Lee Harvey Oswald - Left
Malcolm X - Left — Nation of Islam hitmen - Left
Martin Luther King - Left — Conspiracy of white racists and criminals - Right
Robert F. Kennedy - Left — Sirhan Sirhan - Left
George Wallace - Right — Arthur Bremer - Apolitical Crazy
Gerald Ford - Right — Squeaky Fromme — Left or Crazy?
John Lennon - Left — What's His Name - Apolitical Crazy
Ronald Reagan - Right — John Hinckley - Apolitical Crazy