Some films not already mentioned:
Double Crossed (1917) - Pre-Pre-Code story of corrupt cop coercing dude who stole necklace out of desperation into more thievery to help a politician’s campaign.
The Beast of the City (1932) - Police Chief goes after a mob using non-standard (and excessively violent) methods…and he’s the good guy!
I Wake Up Screaming (1941) - Corrupt cop tries to pin a showgirl’s murder on an innocent dude.
Where the Sidewalk Ends (1950) - Corrupt cop tries to cover up his misdeeds.
Rogue Cop (1954) - Cop on the take turns against the mob after they clip his cop brother.
The Case Against Brooklyn (1958) - Corruption in the NY police force leads the DA to form a special squad of rookie cops to find the corrupt ones.