There are policemen who have anger problems and will kill with impunity when provoked. There are even people who seek to become police so that they can become legal serial killers. I've mentioned it before but I knew a former DC cop, psychopath and serial killer of young black men/children. He loved to intimidate by retelling his murders by mentioning the sounds his victims would make.

Yes there is cultural allowance and protection for killers in the PD but there is also the mentally abhorently ill. The cop I knew was kicked off the force after 3 murders as his only punishment.
(3 'known' murders)