How does a police officer psychological state go from being a great police officer to a horrible police officer after years of being in the service?

Why is law enforcement behavior different from their first encounter they had with someone versus the 20th time?

Is this a psychology issues or is it due to things these officer witness throughout their careers?

Is psychological stress a personal problem for law enforcement officer or is it related problem at home, or is it related to anger management issues?

Why is there a sudden change with law enforcement officers? (Think of the current events that occurred. George Floyd, Brianna Taylor, BLM, etc.)