A genuinely illuminating discussion from sociologist Randall Collins (Penn).   Related posts here and here.

(Thanks to Howard Berman for calling it to my attention.)

UPDATE:  This is a really rich essay, but here's one excerpt to give readers a taste:

Cops are under tension every time they stop a suspect. Tension is higher if conflict has escalated recently by previous incidents; higher if it is a neighborhood with a high crime rate; higher if there has been a chase, or alarming reports over police radio links.

Tension rises sharply when the citizen isn’t cooperative or is defiant. Richard Rubenstein, a sociologist who worked in the Philadelphia police force, reported that the first thing an officer wants in any encounter are signs that the person will not make trouble. He insists on taking the initiative, and controlling the situation in little details, since these are the warning signs for bigger trouble. Donald Black, who pioneered ride-along observations in patrol cars, calculated that the chances someone would be arrested did not depend on race per se, but on whether the person was defiant– and in the 1960s black persons were more defiant to the police (not surprisingly, since this was the era of the civil rights movement). Car chases and running away increase officers' tension even more, since these are also acts of defiance. Citizens who turn their backs and refuse to stop are acting defiantly, even if the initial order was something trivial like “move to the sidewalk” (the first step in the 2014 Ferguson shooting).

Adding together any or all of these factors increases tension. Bodily this is experienced as adrenaline rush, the flight-or-fight arousal. The biggest danger with an adrenaline spike is the loss of perception and fine motor control. When heart rate races to 150 beat per minute or more, fine motor control is lost. An officer may reach for a gun when he thinks he is reaching for handcuffs or a taser. Trigger fingers produce wild or uncontrollable firing. Officers in shootouts report time distortions like going into a bubble, vision turning into a blur or tunnel vision on only one part of the scene. Hearing often goes out so that they don’t hear their own gunshots; voices become incomprehensible. It is a situation ripe for miscommunication and misperception.

Adrenaline-produced distortions explain why shooting incidents happen where it turns out the suspect did not have a gun, or was reaching for an ID; situations where stops for trivial reasons blow up into killings. Since adrenaline takes time to subside, the cop may empty the magazine of his gun, even after the suspect is motionless on the ground. Catching these details on video certainly looks like an atrocity.