The most direct and credible information we have about the motivation of the Benghazi attackers is in this New York Times report by David Kilpatrick: Election-Year Stakes Overshadow Nuances of Benghazi Investigation

To Libyans who witnessed the assault and know the attackers, there is little doubt what occurred: a well-known group of local Islamist militants struck the United States Mission without any warning or protest, and they did it in retaliation for the video. That is what the fighters said at the time, speaking emotionally of their anger at the video without mentioning Al Qaeda, Osama bin Laden or the terrorist strikes of 11 years earlier. And it is an explanation that tracks with their history as members of a local militant group determined to protect Libya from Western influence.

That’s a pretty conclusive statement. The Islamist militants were enraged by the video — as many other Muslims were in the Middle East right then. This group had their own compound, and were already armed and trained. Very little planning would have been necessary.

I don’t think there’s a mystery here at all.