Coercion –- all force –- is unethical. Sometimes force is needed to prevent greater injustices, but even if we need a small injustice to prevent a greater injustice, this does not prevent the small injustice from being ultimately unjust. This is why corruption always occurs in governments, police forces, or active military. Practicing injustice by punishing injustice leads people to practice injustice.
This is clear when we realize what the practical actions of a just police or military force really is. While the presence of police or military force as a threat of force may prevent some injustices, only rarely are they around to actually prevent injustices from occuring. Usually, they react to injustices already performed. Punishing injustices already performed is revenge, and revenge is the returning of evil for evil. This creates a corrupt culture, especially among those with weak characters, as a culture of revenge is a culture of injustice. We create police and military forces to exact revenge for us so the culture at large can be free of the corrupting influence of revenge. This also allows us to act indignant whenever members of the police or military act unjustly –- as we should, even though by putting them in cultures of revenge, we make them prone to acting unjust. Thus, we sacrifice our police and military personnel to reduce corruption in the culture at large.
If we cannot eliminate corruption in a revenge culture, can we at least reduce corruption? The worst kind of good is one that is good because it is better than some other evil. A bad kind of vengeance is one where a greater evil is given for a lesser evil. A better kind is one where an equal evil is given for an evil. But we do not want a police or military with members as evil as those they fight. If we want better, less corrupt protectors, we need laws where, if revenge is needed, the revenge is a lesser evil than the crime. To do this, of course, requires we identify “evil,” including levels of “evil,” so we do not make the mistake of using a greater evil against a lesser one.
A historical example is Prohibition, when buying and selling alcohol could get one put in prison. This was a greater evil (putting someone in prison) for a lesser evil (buying and selling alcohol), and most would now say we used an evil against something that was and is at least morally neutral, since drunkenness and alcoholism are bad (these being actions), though the social benefits of drinking and the health benefits of at least red wine are good. Prohibition gave us dramatic increases in all crimes, especially murder, organized crime, and police corruption. When we repealed prohibition, all the crime rates dramatically decreased. This is why it is important to identify what is good and what is bad, and what is evil, so we can avoid corrupting our protectors.
For more on this idea, I encourage you to read my old essay The Tragic Institutions.