Why is it when Police are shot/killed certain people get happy over the police going down?

When you have a police department that is not fair in their handling of matters it naturally brings about a certain amount of hostility towards them. When a police officer puts on a uniform he lets his power go to his head especially young cops. When a police department fails to adequately do their job or misuse their power this hostility comes out in public. I cant understand why law enforcement fails so badly at improving their reputation . There are a lot of bad cops out there who give police a bad name.