He quite quickly became distrusting of what people say and would adopt a negative analysis of situations. Why? - because for the majority of his shifts he has to deal with people who _are_ lying to him.
Lots of domestic violence calls, drug dealers, thefts, reckless driving, people with serious mental illness (danger to themselves and others). The perpetrators inevitably lie and often the victims are not telling the whole truth (to protect themselves from incrimination for something else or to exaggerate what happened).
Prior to joining he was a well rounded and very grounded person. As far as i can tell, he hasn't resisted the power-tripping trap but I can see some racial and socioeconomic stereotyping creeping in to his world view. It's sad to see but it shouldn't be surprising that people's jobs strongly shape their world view. Some other examples I've encountered personally:
- emergency medical workers strongly against motorcycle riding as they see so many horrific injuries and fatalities from accidents - a child services worker who assumed men interacting with children were pedophiles as her work involved re-homing children who had been abused
For the above occupations, the work involved exposes the workers to highly charged/traumatic situations often daily. A good place to start looking for a solution would be how to help these workers regain a balanced view of society.