<human>: 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." CANNOTANSWER

Answer this question "what did he do as leader?" by extracting the answer from the text above.
<bot>: characterized Yassin as the "mastermind of Palestinian terror" and a "mass murderer." The