Benghazi was the communications center for the transfer of weapons from the Muslim Brotherhood operatives in Libya to the Muslim Brotherhood operatives in Syria (later the Islamic State in Iraq and Syria, ISIS). The Libyan branch objected to giving up the weapons and the attack was launched to break up this clandestine transfer.
Most of the thrust against the long-standing tyrants of the Middle eastern Islamic countries, after the fall of Saddam Hussein, has been with the active participation of the Muslim Brotherhood, of which Al Qaeda was only one of the franchises.