Much like the getaway driver at a bank or someone who knowingly lies to protect his friend from a murder charge, if they police are knowingly protecting the corrupt ones, they aren't the good guys.
If the cops were the good guys, they wouldn't be defending the bad ones and would right with the public on firing them and pressing charges. Once they start lying to protect them or other such things, they cease to be the good guys.
Hell, lying to an officer over a misdemeanor can land you with a $500 fine and 6 months in prison if caught and they wish to pursue to my knowledge (Varies by state). Let alone for a felony, they should be held to that same standard except even more strictly because they are supposed to be the ones enforcing it.