Police are invested with a degree of authority ordinary people are not. When they commit a crime, not only are they breaking the law, they are often abusing their authority, an additional offense.The law says something different, like, you can't be charged for violating someone's civil rights, because you have no duty to protect civil rights, like a sworn police officer or an officer of the court. The law holds those to a higher standard.
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It is an additional offense but they are judged on the action or crime like any other citizen. Police Officers are given authority to enforce out laws. They are able to do things a regular citizen would be charged for.
Impersonating a police officer is a crime in itself. For a police officer to abuse that authority is a seperate crime.