I don't think that's why he was killed. He was killed because he was an American ambassador and in the middle east they kill ambassadors.
I think he was allowed to be killed because it was part of an entire course of action that started in Egypt to increase violence in the region. The purpose of the increased violence was to force the US to release Shiekh Rahman as a gesture of good faith. That came up soon after Benghazi happened.
Was Benghazi A Set Up To Release The Blind Sheik? - The Brenner Brief | The Brenner Brief
I don't believe Ambassador Stevens was to be taken alive. The deal would have been to release Rahman as a gesture of good faith to stop the violence.
