As far as I can see, the only party that benefits from Gemayel's assassination is Israel. Israel was the main loser in last summer's war, at least politically and strategically. The country's leaders began threatening a new round of fighting even before they began pulling troops out of the south of Lebanon. Hezbollah's post-war ascendence was the most visible and troubling sign of Israel's seemingly unprecedented military weakness and strategic blundering.
Pulling off an assassination like this, which is by no means beyond Israel's ability, would serve several goals: First, it would turn the chaos that Hezbollah was trying to create in the Lebanese political system against it. Instead of Hezbollah managing the post-war chaos in order to strengthen its position, the movement is now forced onto the defensive and must react to a new dynamic in which Christians (with the exception of the breakaway Aoun faction) and Sunnis are more united than ever in their desire to block Hezbollah's takeover of the system. Second, if the country descends into civil war, which is a frightening, if still distant possibility, Hezbollah would be effectively neutralized, and Israel could rely on Maronites and perhaps Sunnis to attack Hezbollah without Israel facing the international condemnation it received during the war.
If we're so smart, why can't we subjugate the world?