Background: Ahmed Yassin was born in al-Jura, a small village near the city of Ashkelon, in the British Mandate of Palestine. His date of birth is not known for certain: according to his Palestinian passport, he was born on 1 January 1929, but he claimed to have actually been born in 1937. His father, Abdullah Yassin, died when he was three years old. Afterward, he became known in his neighborhood as Ahmad Sa'ada after his mother Sa'ada al-Habeel.
Context: Yassin was a founder and prominent leader of Hamas, which is regarded as a terrorist organization by a number of national governments. Israeli Prime Minister Ariel Sharon characterized Yassin as the "mastermind of Palestinian terror" and a "mass murderer." The Israeli government repeatedly asserted that Yassin was responsible for a number of terrorist attacks, which targeted and killed a number of civilians. They accused him of being behind all the attacks perpetrated by Hamas against Israel. Israel said the targeted killing was in response to dozens of suicide attacks by Hamas against Israeli civilians. According to an Israeli government website:  Yassin was the dominant authority of the Hamas leadership, which was directly involved in planning, orchestrating and launching terror attacks carried out by the organization. In this capacity, Yassin personally gave his approval for the launching of Qassam rockets against Israeli cities, as well as for the numerous Hamas terrorist bombings and suicide operations. In his public appearances and interviews, Yassin called repeatedly for a continuation of the 'armed struggle' against Israel, and for an intensification of the terrorist campaign against its citizens. The successful operation against Yassin constitutes a significant blow to a central pillar of the Hamas terrorist organization, and a major setback to its terrorist infrastructure.  In his statement Yassin declared that Hamas did target Israeli civilians, but only in direct retaliation for the death of Palestinian civilians. In his thinking this was a necessary tactic to "show the Israelis they could not get away without a price for killing our people."
Question: what did he do as leader?

Answer:
characterized Yassin as the "mastermind of Palestinian terror" and a "mass murderer." The