This statement seems like a false bias to me, particularly given that pretty much every policeman/policewoman that I've known personally have been humble and good people and became police either on a desire to help people, or just because it was a paying job.
My evidence is anecdotal, but what is your evidence based on?
Here is a really brilliant Reddit comment that covers a broad survey of studies that broadly support my point: https://www.reddit.com/r/AskSocialScience/comments/b9fkny/is...
I'd argue though that working in a profession that is inherently violent at times and in which you see the worst in people can't be good for your mental health, not to mention potential PTSD from some of the things that police witness. Family abuse is absolutely not good and should be looked into/stopped, but correlation doesn't necessarily mean causation.