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In all, 582 people were killed in the storming of Damour.

I was unfamiliar with the massacre at Damour so I looked in up. Unquestionably this was a terrible incident but you omitted two important facts:

1. It was during the Lebanese civil war.
2. This massacre was in retaliation for a massacre committed two days earlier by Christians against the Palestinian town of Karantina during which even more people (~1000) were killed.

With all my respect to Wikipedia, a block of 300x300 meters could not have suffered 1000 persons killed. The number of 70 killed, reported by the newspapers of tuesday Jan 20, 1976 is more reasonable. However, Karantina held the 2 bridges over Beirut river under sniper fire for more than 9 months, causing several civilian casualties every day.
Karantina, Damour and many more places suffered heavy losses in January 1976 during a general offensive conducted by the palestinians and covering about 75% of the lebanese territory going from Jezzine in the South to Tripoli and Zghorta in the North, cooling down only to permit the visits of Mr. Khaddam (yes himself http://www.freerepublic.com/focus/f-news/1552808/posts) and syrian general Shehabi to the presidential palace to present successive demands to the lebanese authorities. The same newspapers reported in the same day, the entry of two palestinian brigades into the lebanese territory coming from Syria, the first from the East into the Bekaa valley, the second from the Northern border.
All this and the reasons of it, is duly explained in the speech pronounced by the Syrian president Hafez Assad in the University of Damascus on July 20, 1976.
As you say "Unquestionably this was a terrible incident", but that could not explain the official position of the White House whose spokesperson declared (reported in the newspapers of Jan. 20, 1976) that Lebanon should be able to solve his problems by himself!
You might want to ask me how do I know that? Well, I lived it the very tough way.