4 ideas
Moose Malloy, a giant ex-con released after eight years in prison, goes looking for his old girlfriend Velma Valento. He accidentally kills a man at the bar where she once sang, and private detective Philip Marlowe gets pulled into the search. Velma has become Mrs. Helen Grayle, wife of a wealthy Bay City man, and when Moose finally finds her, still devoted to her, she shoots him five times and later kills herself rather than stand trial.
Police corruption is structural, not a matter of individual character. When a town's money comes from gambling, vice, and rich patrons who pay for protection, even an officer who wants to stay clean has to go along or be pushed out.
Someone who climbs socially by erasing their origins turns everyone who remembers the old identity into a liability. The more the new position is worth, the more pressure there is to silence those people, through payoffs, intimidation, or murder. Following who is being paid or eliminated reveals what someone is hiding about where they came from.
Illegal enterprise settles where authority is fragmented, such as a small, separately policed beach city or gambling ships anchored just past the three-mile limit. Each boundary lets officials on either side disclaim responsibility, and reaching the operation requires going around formal channels. Corruption survives through these seams between jurisdictions as much as through bribes.