If the "dozens" of attackers, who we know had AK47s, RPGs and light artillery, had simply intended to kill the ambassador I'm pretty sure the entire unguarded staff at the consulate would be dead and there'd be evidence of gunfire and high explosives.
We do know the terrorists attacked the CIA annex with gunfire and artillery.
What were the attackers' objectives at the consulate?
I would guess it's the same motivation the Taliban has for attacking U.S. installations in Afghanistan. They want the U.S. out. Earlier this year the British post in Benghazi and the Red Cross operation were abandoned after being attacked. The U.S. consulate and the CIA post were probably seen as obstacles to Al Qaeda or their Libyan cohorts from taking over.