The officer said he had thought the autistic man had a firearm.
The officer said that he thought the autistic man was a danger to himself and/or to public.
So the officer discharged his firearm and accidentally shoots the black care worker.
Here is why this explanation makes no sense.
The autistic man still had the toy truck (which is seen right next to him after he was cuffed). The officer still manages to get out from behind the cover and approach the two men. This seems to support that he was not afraid of the autistic man and he knew it was not a firearm.