When we talk about illegal activities ,those can vary from not paying the taxes till genocide so the margin is quite large here. Assuming we’re talking somewhere in the middle, if you know that a police officer is involved in something illegal you should file a complaint.
Because regardless if people people know,an investigation must have a legal reason to start. I know many police officers involved in all sort of dirty stuff. I choose to not fill a complaint because it’s not really my business and their activities doesn’t affect me .the issue is that when “people”know, the term people involve also coworkers, neighbors and family.
There are great chances that a police officer breaking the law will get away without any problem. here are some photos that shows how police officer violate public rules and misuse them for there own use.