When I looked at the video, and listened to and read comments about what happened, I thought you were being very, very generous to BE who had been "trolling" behind the victim, Sandra Bland (she was the victim of a violent arrest don't forget) and forced her to make the lane change so that he could have grounds to arrest her.
BE, IMO, had already decided he was going to make an arrest and get his quota made, and there was nothing Sandra Bland could have done to change it.
IMO, BE was looking for a response, and if he didn't get one through his nasty, unprofessional manner, he'd force one physically.
Sandra Bland was polite to him while she was in the car. He forced the issues, and he was not going to let her go. No matter how polite she was. It would not have mattered. He did not follow his training or correct procedures.
BE forced her to change lanes, thereby starting the incident.
BE unlawfully told Sandra Bland to put out her cigarette.
BE unlawfully demanded that Sandra get out of her car without giving her a reason.
BE refused to tell Sandra the charge on which she was being arrested.
BE held a taser to her face and threatened to light her up. If that thing had gone off accidentally she would, at the very least, have suffered catastrophic brain damage. He was not using a dangerous weapon in an authorized manner.
BE changed his story about the manner in which he claimed he had been assaulted.
BE slammed Sandra's head into the ground.
BE hurt Sandra's wrist.
When told of Sandra's medical condition, BE said "good".
IMO, Sandra was angry and frightened, and, after the recent series of situations in which unarmed black people have been assaulted (a 14 year old girl in a bikini was considered a threat by one gun carrying cop) or killed by LEOs, few could blame her. I don't think it's reasonable to expect someone who has been tailgated, bullied and told she is under arrest by a man who will not tell her what she is being charged with to just be cool and polite.
Perhaps BE should not have bellowed demands or shrieked that he was going to light Sandra up with his taser. I certainly don't think that made him a particularly powerful figure either.
Sandra Bland did not attack BE. BE is not a victim.
Sandra Bland was the victim.
BE instigated and escalated this fiasco every step of the way because, IMO, he wanted the result: meeting his arrest quota.
However, I will agree with you that someone at the scene could have and, I think, should have kept her cool and de-escalated the rapidly deteriorating situation.
She had the knowledge, the skills, the power and authority of her badge to do it.
And that person utterly failed to do what she had a duty to do, had been trained to do, and was being paid to do.
BE's partner failed to do her job. By not by not upholding the laws as she was sworn to do, IMO, she let down Sandra Bland, she let down her department, and she let down the citizens of Waller County.