Asassinating Sheikh Yassin was justified, even if he himself did not personally kill Israelis:
Via Anne Cunnigham and Oliver Kamm, I came across the following paragraph in an essay by terrorism expert Walter Laqueur which sums it up nicely:
The suicide terrorist is only the last link in a chain. There is no spontaneous suicide terrorism. The candidates are chosen by those in charge of the organization. The suicide terrorists are indoctrinated and trained - receiving intelligence information to guide them - and eventually are given the arms and explosives to carry out their mission. The people who guide suicide terrorists have their political agenda. They organize the missions not as a purposeless manifestation of despair but to attain a certain political aim. While the suicide terrorist may be unstoppable, those behind him are certainly not; they can be deterred by inflicting unacceptable damage on them. Thus the leadership of the Lebanese Hizbullah after years of suicide terrorism [a tactic that it initiated in the early 1980s] discontinued these operations realizing that they were no longer very effective.