Often the main thing with a job is that it be done well, and the practical distinction between the quality of separate workers is how well they do a job.
For example, given a choice between hiring two builders: an adulterous boozing reprobate who builds excellent houses, and a churchgoing paragon of virtue who builds houses that shake in the wind; many people would rationally prefer the reprobate.
Why then are policemen generally required to first attest to being and remaining uncommonly virtuous, (even when off-duty), as a primary duty, rather than being mainly valued for their relative skills in the tasks of policing**? It seems like making personal virtue primary might tend to maximize hypocrisy and corruptibility.
What's the evidence that background checks and oaths are useful in this field? As opposed to letting anybody join up, and regularly winnowing out those who do a bad job.
** Let's suppose that good policing begins with one police officer who does a good job. That is, an officer who ably selects what to enforce sensibly, enforces laws impartially, doesn't slack off, avoids corruption and on-the-job deviancy, is not a reckless driver, and where violence is needed quietly shuns personal inefficiency and theatrics. Good policing is more officers like that, and fewer of their opposite.