Neither the threat of executions nor life sentences deterred these cop-killers. Are they that stupid?
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They are as different from us as sharks are. They see an opportunity, and they take it. I don't know if there is newer research, but back in the early 90s, there was a report from the FBI on studies of those who assaulted/killed cops. They overwhelmingly said that they did not try to take on the ones who looked hard, who conveyed the impression that they were the apex predator (and never doubt for a minute, cops are supposed to be hunters, looking for bad people and doing what it takes to control them). They took on the ones who looked like "nice guys". This is also consistent with the studies of who criminals chose as prey. (Even DOJ/FBI can do the blind pig and acorn thing, probably about as often.)
At the department at which I worked at the time, we all knew and agreed who would be the one to get attacked or killed. He had good, even excellent skills with regard to suspicion and observation, but bad to non-existent skills with regard to control and safety. He thought (and regularly documented) that a couple of us were too hard, too aggressive, too safety driven, etc. Oddly enough, after I moved out here, I got a printout of the dispatch from an incident in which he proved what we thought. He was nearly beaten to death with a crowbar by a guy he caught burglarizing a business. His first part of that was well done, but he did not wait for backup, and did not hold the offender at gun point.
Fundamentally decent people, who of course generally know nothing of pathology/bad people, think that's what cops should be. The enablers of criminals, who think we kill and hurt too many (and the research data clearly contradicts this), think so too. The offenders laugh at them, and continue being bad actors. Because cops have been increasingly savaged in the press for inane, perfectly lawful uses of force for a few years (and I am not saying cops are perfect, either), agencies and officers have been far too hesitant to use force to impose control when needed. Heck yeah, it looks ugly. So? That's not the standard.
Sorry for the drift, but it seemed necessary for the moment.