Windriverrange,
The attack was definitely pre-planned and carried out by a group which was an al-quada affiliate.
What I relayed was what the leader of that group said was one of the primary motivating factors that caused them to attack. Ahmed Abu Khatallah, the ringleader and apparent mastermind behind the attacks, who was with the group Ansar al-Shariah. He was captured in 2014 and brought to the US and he currently sits in Jail in DC, awaiting trial.
He was the one, who while still under military custody said that the videos were part of the reason why they attacked.
Gen Carter Ham testified before Congress that Amb Steven's had been offered additional security at the Annex and Consulate just weeks before the attack and had turned down both offers.
http://www.military.com/daily-news/2013/05/15/official-amb-stevens-refused-additional-security.html
From other details that I've read, Stevens previously had asked for additional security for the annex and consulate numerous times previously and there was no budget for it. The original plan was to provide some 5 US "Contractors" to be available to defend the annex and consulate building in Benghazi as the agreed upon number, but there were seldom 5 there at any given time. In addition to those contractors (most ex-Seals), there was, just like nearly every other country where a US consulate is located, the US Govt had contracted with local security forces who were supposed to provide the bulk of the fighting.
That day/night, the local forces did actually show up and help fight the attackers. They are the ones who retrieved Stevens from the building and took him to the hospital and apparently later brought his body to US officials.
But bottom line, Stevens knew full well that Benghazi was becoming a much more dangerous place and the provisional Libyan government did not have a full handle on the city and security was eroding. Yet, he turned down the additional security measures African central command offered and traveled to Benghazi when he knew it was a dangerous decision. Some times, brave men make brave decisions, that they end up paying with their lives.