Back in the late 70s, Professor William Muir of Berkeley wrote--Police: Streetcorner Politicians. It is to my mind one of the best books ever written about policing. It explains clearly why the average police officer enters so many encounters aggressively and resorts to violence so quickly.
As Muir notes, police face two types of people: the dangerous and the not dangerous. Police can also enter an encounter in an aggressive, domineering way or as a helper or negotiator.
If people are not dangerous, and police are not aggressive. All goes well. If people are dangerous and the police treat them as such from the beginning of an encounter, then the police minimize their risk of injury. In these instances, the interactions are symmetrical, and the police response is appropriate for the circumstances.
Problems arise when asymmetry occurs. Someone is dangerous and police don't recognize that danger. The result may be an injured or dead officer. If a person is not dangerous and police treat them as such, then you have an unhappy citizen, possibly one who is either humiliated, injured, or killed.
Police are risk minimizers. Many police believe best way that they can minimize thier risk of injury or death is to enter almost all encounters in an aggressive or domineering manner and move to violence quickly, so that they protect themselves and maximize their control over a situation.
So, how is it that encounters over selling loose cigarettes or walking in the middle of the street end in civilian deaths? Police have the authority to tell people to do all sorts of trivial things. At times, they baldly do so, and some people fail to comply immediately or quickly enough. That failure to comply is a crime.
The officer, having given an order, is then faced with a serious dilemma. Does she or he back-off and let the civilian flaunt a lawful order, serious or silly and potentially become a "threat" at some later encounter because the citizen has earlier been successful in defying the police. Or, does the situation demand the police involved immediately exert all the power, potentially lethal force, needed to support their claim to authority and their ability to demand subservience.
The old, but important, saw 'Don't draw your weapon unless you are willing to kill' has a corollary: when you are not at risk, don't enter an encounter over a trivial matter by giving orders that must be obeyed immediately or they place the civilian in the position of committing a crime by disobeying a legal--but meaningless, unfair, or offensive--order.
Talk is a vital weapon for a police officer. It is the weapon of first resort and must be used liberally. But, most police aren't trained to talk in ways that may defuse a conflict or a dispute and avoid violence.
Cops are taught how to fight, and they are taught that "Cops do not lose fights." But, since police are almost always willing to use the full force at their disposal to win any fight they enter, that means police must avoid fights if at all possible.
Police need to be trained to use good judgment about deciding to give an order or enter an encounter that may be conflictual. They must always consider that decision in terms of how far the officer is willing to go to assure compliance, and how conflict can be avoided.
Is walking in the middle of the street or having an arrest record for selling loose cigarettes worth a conflict where an officer believes he might feel forced to use maximal or lethal force.
Police must be trained to consider what dominance in an encounter is really worth? And, is it worth the price police might make others pay, and they might pay themselves, when they value it more dearly than anything else?
Of course, all this applies only to those officers who take "protect and serve" as their motto. There will always be officers who seek conflict or violence, and who enjoy the exercise of power, for whatever reason. For these police, the only answer is a request that they not let the doorknob hit them in the ass as they leave the profession.