The video suggests the killers were well organised, had a plan, and were confident of their success and escape. They pause to yell "We have avenged the Prophet Muhammad!" before driving on.
It also shows that a Paris police car, which ostensibly could have trapped them at the scene by blocking the street, was outgunned by the terrorists who fired on them from their moving vehicle, forcing the police to retreat.
The video was shot on a mobile phone by terrified witnesses cowering on a roof above the scene. They speak French, and at one point say they cannot believe their eyes, that the bullets can't be real. It becomes obvious that the bullets are real when the police car slams into reverse as the two killers, brothers Said and Cherif Kouachi, advance on them.
Here is the video. It does not show any blood or killing, but it does show scenes of shooting and violence from two men who have just killed an office full of cartoonists. As such, be warned - it is disturbing.