Here is what I think happened.
During the overthrow of Khadafi, guns and assault weapons were provided to the rebels. The American public was not aware of this. These rebels turned out to be al qaeda and realizing the weapons were in the hands of terrorist, the administration wanted the weapons back, and sent CIA in to get them.
The rebels said scr*w you Obama. The administration had information that an attack on the compound was imminent. They didnt tell the embassy. Obama was hoping that the compound would come under seige and that the staff would be taken hostage, much like the Iranian crises. Unlike Iran, Obama thought that he could negotiate the release of the hostages and thus look like a star.
That explains why the terrorist were led to the safe haven by staff insiders. What Obama didnt count on is the staff of ex-Navy Seals fighting back with all their ability.
Keep in mind that Obama was well aware of what was going on. According to the White House website, below was Obama scheduled that afternoon 11Sept2012. He was in the Oval office meeting with his Def Sec Panetta when this attack went down.
8:45 am || Observes a moment of silence to mark the 11th anniversary of the 9/11 attacks; South Lawn
9:30 am || Attends September 11th Observance Ceremony; The Pentagon
2:15 pm || Visits with wounded warriors who are being treated at Walter Reed National Military Medical Center; Bethesda, Maryland
5:00 pm || Meets with Secretary of Defense Panetta; Oval Office
You ASSume the White House schedule reflects reality.
I bet Obama was nowhere to be found.