Why was King Abdullah of Jordan assassinated in 1951?
King Abdullah, alone of all the Arab leaders, was a moderate in the eyes of the West. He even stood for peace with Israel, and would actually have signed a separate peace agreement but for the Arab League's militant opposition. On account of this, and because of his dream for a Greater Syria comprising Jordan, Syria, and Iraq under the Hashemite dynasty, many Arab countries distrusted Abdullah.
On July 20, 1951, while visiting the al-Aqsa Mosque in Jerusalem, King Abdullah was assassinated by a Palestinian extremist afraid that the old king would make a separate peace with Israel. The gunman fired three fatal bullets into the King's head and chest. Abdullah's grandson, Hussein Ibn Talal (King of Jordan from 1953 to 1999) was at his side and grappled with the assailant until he was shot himself. A medal that had been pinned to Hussein's chest at his grandfather's insistence deflected the bullet and saved his life.
King Abdullah was one of three relatively moderate Arab leaders who were targeted for assassination because of thier views. Middle East politics has been dominated by extremists whose hate is so strong that they will kill their own leaders to prevent any progress toward a peaceful solution. For an example on the Israeli side, read about the assassination of Yitzhak Rabin.