Sadly people forget what the adage means. When you have a bad apple in the barrel it spoils all the other apples, the good apples become bad apples by association. And that is what we see in police forces. There may well be cops that don't do anything bad themselves, but they don't stop the bad apples even when they know the bad apples are doing wrong, which means they are complicit in the wrongdoings, therefore they are also bad apples.
I am sure there are otherwise good people in all police forces but if they don't tackle their colleagues when they see or hear of any wrongdoing then they are bad police officers. And once that barrel is spoilt it will remain capable of turning good apples bad no matter if you have thrown out all the current bad apples.
To stop torturing the adage: Police forces should be policing themselves even tougher than they police the public, they should be squeaky clean internally, it means there must be proper checks built into the police procedures that stop police officers being able to do and get away with wrongdoing. Technology can now help this; it is entirely possible for example to create a system in which the complete "chain of custody" from arrest to court appearance can be reviewed from sources the police can't tamper with. Police on duty should be GPS tagged. There are lots of ways we can make it hard for bad apples to start to spoil the barrel, it is now a political question, are our politicians willing to do what is necessary?