The reason Stevens was meeting with a Turkish diplomat was because they were smuggling black market weapons from Libya to Syria, through Turkey.
The CIA annex was where the two men who eventually got involved in the gunfight were at. They heard gunshots, asked for permission to go help, and were ordered to stand down (by "someone"). They eventually disobeyed orders and went anyway.
Barry and Hillary were watching it LIVE via drone camera.
This all came out a few years ago (except for the smuggling part, but that is obvious since the fake attacks in Syria were done with weapons that were traced back to the stockpile in Libya).