On the one hand, the police often have to do a dirty job and deal with many nasty "clients" at the more unpleasant underbelly of society. In short, they do a job which not many of us would like to do. ("Where are the police when you need them?" Well, as we all know, the police cannot be everywhere.) Not surprisingly, doing a job like this requires a certain pyschological toughness that not many of us possess.
On the other hand, this pyschological toughness usually turns into into a mentality which might not inaccurately summed up with the idea that "To maintain the law, you sometimes have to break it." I will not debate the validity or otherwise of that idea here.
In other words, many such police officers the world over believe (a) they are the goodies, (b) that justice is (or should be) what they personally believe it ought to be (a bit like many of us, if truth be told) and (c) that for that reason they themselves are or should be above the law. This is what gives rise to the contempt towards and the feeling of betrayal towards "whistle-blowers" who report breaches of the law by colleagues - usually at the price of their own careers.
In a country with a legal system based on the rule of law, this violates the principles of observance of the law by all (including the police), neutrality of the law and equality before the law. Such violation is a massive, massive mistake and arguably represents a threat to both the legal system and (indirectly) to democracy.
Bring on the downvotes.