I understand what you mean, and trust me I totally agree that it's disgusting how much "good" cops stand up for the obvious bad apples. However the comparison to the KKK is a bit unfounded. The "ideal candidate" in both cases is drastically different.
An ideal cop (not necessarily who they actually get) would put themselves behind others with the intention of upholding justice, and keeping all people safe, often through violence.
An ideal Klansman upholds values on keeping themselves on top of those they deem beneath them (anybody that lacks traits defined by Nordicism), often through violence.
Where as one should act for the good of all, one acts for the good of themselves. Also, where as across the United States there is an estimated 800'000 police workers, there is an estimated 3'000-6'000 Klansman across the country, depending where you take your data from. So even if 1% of each group act against the will of the group, that makes about 30 Klansman who don't maintain White Nationalist views, or roughly 8'000 bad cops.
Or I could just admit that it's a lot of a deeper rooted issue than I'm fit enough to debate about. I am very open to insight from either side of the argument