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Citizens in the US have a duty to de-escalate the situation, a 'duty to retreat', unless they're backed into a metaphorical corner ('castle doctrine').
Police are presently seen as having a duty to escalate - to allow someone potentially hostile to back down and leave without handcuffs is seen as a dangerous failure, extending even to periods when the police officer is off-duty. Meek compliance with 'lawful orders' is the penultimate goal, and people will be bossed around, arrested, tortured (Who the fuck thought 'drive stun' mode was a good idea), or shot for failing to show appropriate amounts of submissiveness.
Assault against a police officer is seen as a crime against the state, whereas assault against a citizen is essentially mandated for a police officer to do their job.
The rules for actual murder are only slightly less assymmetrical.
Examples abound.
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First breaking off civil relations with the citizenry via the drug war and then paramilitarizing our police force post 9/11, and finally having their behavior revealed with Youtube and smartphones, has severely damaged the credibility of the police in this country, good and bad; It's going to take some severe changes to bring it back - changes explicitly designed to "make it harder for them to do their job", as they would describe it.