Those in the video were looters who moved in quickly after the assailants left. They unexpectedly came across the ambassador and alerted the Libyan security people who had moved back in.<quoted text>
Who told you they were "cheering"???
I hear a lot of shouting and see concerned faces. If they were "cheering" wouldn't you expect more smiles?
Somebody told you they were cheering so that's what you see because you're a good little soldier in the fight for what's right (no matter how wrong it is).
What you're seeing in that video is a couple of Libyan security men bringing the body through a group of young looters. What you're not seeing is a vehicle just out of frame and of course weapons.
WSJonline and others have produced fairly accurate depictions of how the entire event unfolded.