Hamas IS a terrorist organization, the US revolutionary movement was not. It's not to say the Americans followed all the rules of war during the American Revolutionary War, but the overall process and goal of the Americans was not to terrorize the British civilian populations in the colonies, but to militarily defeat the British armed forces in the colonies.
The goal of Hamas is not to defeat the Israeli military, but to terrorize the general Israeli population to such an extent that the Israeli political leaders no longer have the backing of the people required to continue the military occupation of Palestinian areas and are forced to retreat.
The objective of the American revolutionary forces and the Palestinian terrorists might be very similar, but the ends to the means are vastly different.
The assignation of the Hamas leader was not an act of terrorism by Israel. Ahmed Yassin was a self declared and recognized leader of Hamas and ordered the deployment of dozens of suicide bombers resulting in the deaths of hundreds of Israelis. Even if you argue that Hamas is not a terrorist organization, but a valid army, the assignation is still fair. Although many countries now condone the use of assignations to hinder an enemy, it is not a terrorist act. It is considered legitimate and has been used many times by sovereign nations in the past as a method of war.
Now some other things that Israel has done I think can border the definition of terrorism. Bulldozing civilian housing in retaliation for suicide bombers seems an awful lot like going purely after civilians rather than attacking terrorists.