Genre cross-overs in most entertainment can work incredibly well, such as horror comedy. The reason that this works so well is that is that horror and comedy are two sides of the same coin, they both rely on the unexpected for their big pay off, and so mould well together. Not so much true crime and romance. Sure there are couples in true crime, Bonnie and Clyde, Starkweath and Fugate who Natural Born Killer's Micky and Mallory was based on, and later Iain Brady and Myra Hindley. While there are a few big names, couples where both are active in a crime tend to be the exception rather than the rule, but even more rare are couples where one is a criminal, and the other is a serving police officer.