Think about the position they are hired for.

They are paid not fantastic wages, and EVERYONE hates them. There are only 3 types of people who would apply to a job like that.

A) A person who is power hungry/enjoys the idea of being in control. These people tend to be the corrupt ones.

B) Those with a genuine dedication to justice or want to help their communities. These people tend to be the ones who are scapegoated for things by the type A.

C) Those who need money badly and have limited options/experience/training. These people are also easily corrupted as they already are desperate for cash.