Justice should be about telling "the truth, the whole truth and nothing but the truth," and that truth telling should apply to both sides of the law, so that true justice can therefore be rendered. One can guess that it almost goes without saying that the defendant in a criminal case, along with witnesses selected for the defense, are probably not going to tell the whole truth. However, it must be said, that the prosecution side typically does not disclose the whole truth, itself; and the prosecution, is specifically suppose to represent not only the state, as well as the people, but also fair justice, applied.
In regards to the prosecution, the most problematic part of their truth telling often comes down quite tellingly to the real motivation of some of the witnesses so presented. For instance, any testimony in which the person testifying has been explicitly or implicitly threatened by the prosecution, then such testimony as that, is surely not the whole truth, but rather something that has been coerced or coached, or both. Additionally, those that testify in which as a condition of their testimony they will receive leniency so as to preclude incarceration, or the length of such, or the threat of such occurring, are obviously going to testify in a manner in which they are going to say what they say that favors their own personal interests in conjunction with those that they are obviously beholden to. Finally, there are plenty of people that are provided with monetary funds from the government or its authorized agencies or personnel in one form or another, in order to entrap, corrupt, instigate, inform, or anything else along that same some of line, so as to get the desired ultimate result, which clearly cannot conceivably be: justice.
While the United States judicial system prides itself that it never bribes anyone in any way, at any time, the fact of the matter is, that the United States does exactly that, in far more cases than it so desires to own up to; for every time that testimony is purchased on behalf of favoritism so shown to a particular person so as to produce that necessary testimony, that is, in essence, bribery. Additionally, in a fair judicial system, all deals so agreed to with witnesses for the prosecution, should be transparent as well as being fully vetted and thereby disclosed to the other side, along with such being disclosed to the jury, so that such testimony can thereby be appropriately weighed, or alternatively to that, such testimony which has been purchased, directly or indirectly, in order to assist the prosecution, should be subject to an outright ban.
After all, people that are employed, are paid for their work; and all those in the judicial system that are paid in one form or another, for their work, or for their work to come, are going to, typically, do what they need to do in order to justify having been paid. So, it can be said, that all the games that prosecutors play, in order to secure a conviction, of which the prevailing attitude is often one of whatever it takes to get that conviction, is never going to be justice served, but rather will always be justice denied.