When police officers act in unethical ways, it reduces the amount of trust that the public has for the police and the law enforcement system in general. This is a very damaging thing for the relationship between the public and the police.
A good example of this can be seen in a recent scandal in New York City over police officers "fixing" traffic tickets. When things like this happen, the public starts to think that the police do not always treat everyone equally. They think that the level of justice that a person gets depends on who they know or other such improper factors. This makes them trust the police much more than they otherwise would.
In this way, unethical behavior by police lowers citizen confidence in the police and harms the entire community.