I outline below a hypothetical that poses a moral dilemma for an imaginary public prosecutor. This case vexes me, because I cannot seem to summon up a convincing argument that supports my intuitive reaction to it. I would love to hear the perspectives of other philosophers.
Suppose you are a state’s attorney assigned to prosecute a heinous crime, say an especially horrible murder, with multiple aggravating circumstances. The defendant has been previously convicted of a number of violent felonies, so you have good cause to believe that if he is not executed or put away for life, public safely will be seriously jeopardized.
You have an overwhelming case against this defendant, sufficient to convince any neutral observer of the defendant’s guilt beyond all doubt. There is just one, potentially fatal, flaw. An absolutely key piece of evidence, without which you cannot proceed, was obtained in a manner that would, under prevailing judicial precedent, require that it be excluded at trial. There is nothing in these circumstances, however, that impugns its reliability as proof of the defendant’s guilt.
The “good news” is that you are the only one in the universe that knows the evidence is procedurally tainted. Perhaps the detective who secured the warrant leading to this evidence disclosed the infirmity to you, and has since died, and you are as sure as you can possibly be that this “technical” flaw will never come to light. The legal standards governing prosecutorial conduct are clear: you are required to disclose to the defense all exculpatory evidence, which will result in it being excluded, and the defendant walking free. Should you comply?
I am troubled by the prospect of using the evidence, but am struggling to formulate a good argument to support my concerns. One obvious rubric for analyzing this dilemma is whether “the ends justify the means,” meaning whether a bad act is permissible in the service of a greater good. The obvious answer for a libertarian is “no,” because we are not permitted to treat persons solely as a means of achieving some supposedly greater good.
But this case seems different because we have by hypothesis a guilty person who deserves punishment, and who arguably has forfeited all procedural rights by his horrific crime. One plausible view is that these rules are in place to ensure that justice is done, i.e. to protect the innocent, and not to shield the guilty.
There is also the issue of whether agents are entitled to disregard at their discretion the cannons of legal ethics, when these rules that have a just purpose and are generally beneficial in application. There is something troubling about this violation, but I don’t see any obvious reason why rule worship is required in this case given the huge utilitarian cost. Why not ignore the rules when they would produce a terrible outcome?
So, I am left with the conclusion that the state’s attorney should violate his putative duties in these circumstances, but am very uneasy about this. There is probably a lot I have missed, and would appreciate your thoughts.
 See Lisa M. Kurcias, “Prosecutor’s Duty to Disclose Exculpatory Evidence,” 69 Fordham Law Review 1205 (2000), http://ir.lawnet.fordham.edu/cgi/viewcontent.cgi?article=3689&context=flr