He was beat up pretty badly
I'd heard he claimed the young man broke his eye socket as he punched the officer through the window of his cruiser.
Even still, had he been injured, as cops often are on the job, he's not entitled to drive himself away from the scene where he just dropped a civilian by firing 6 bullets into him.
And with an alleged broken eye socket, his driving would be compromised due to vision impairment, and of course shock.
That's why a cop is also to remain at the scene after calling for backup and assistance. So that an ambulance can be dispatched to attend the injured, including himself, and others who may be in need of help.
But this officer apparently not only left the scene, but never called in the shooting. While dispatch, recorded and posted on YouTube by someone presumably with a police radio, is heard talking about this officers shooting encounter because they heard about it on the news. Not via his call in to dispatch to report he'd just shot an 18 year old dead in the street of Ferguson.
Apparently the cops that did respond to the body of Mr.Brown were called there by the residents. There is a cell phone video on You Tube that shows Mr.Brown dead in the street after the fact.
There isn't an officer anywhere close to him as far as a vehicle that would have been the officer in question remaining at the scene. It's graphic due to the body of Mr.Brown being fully visible, not pixeled out as in other videos on YT. And due to the graphic language of residents who were either live witnesses to the actual shooting, or arrived shortly after the fact.
You can find it and view it at your discretion by going to YouTube and searching for the video title:
phone video of Michael Brown shooting
It's horrible to see him laying there like that. I know he's dead but I even felt compelled to help him from the other side of this monitor. To see someone like that, blood running from the top of their head and down the street, and cops are keeping his uncle away, a man you'll see in a black shirt trying to get to Michael, before a cop runs and stops him. While Mr.Brown's mother, who is wearing scrubs and is seen on the grass across from her son, is also being held back by neighbors and police due to her son's dead body now being part of a crime scene. It's heartbreaking. And there is no weapon anywhere to be seen near his body.