Korinda Martin was a jailhouse informant. She was one of three main groupings of evidence the prosecution tried to use to bolster their case against Kirstin Lobato (the other two being Kirstin's statement to the police and a shoddily woven tapestry of circumstantial "it's possibles"). But there are a number of reasons that I believe that the use of jailhouse informants violates the constitutional right to a fair trial.
It actually doesn't go to character too much. People to tend think that "criminal = total badness," when the truth is that they could certainly have positive characteristics despite their choices. So, by itself, the preconception of criminals being liars is not going to be well founded in all cases. Nevertheless, they are often given what they would see as good reasons to lie.
For one, they can hope that by aiding the prosecution, they aid themselves. While no prosecutor in their right mind would ever promise to help out an informant prior to the trial, they can certainly imply it without saying it. And this is rooted in a simple value system. The things prisoners value are few: intangible items such as power, prestige, and survival. But it is freedom that probably ranks the highest...and the sooner, the better. And as I said, the prosecution won't openly make this promise. Almost every case involving a jailhouse informant involves that person assuring the jury they have been promised nothing, even as they knowingly screw over another person in an attempt to cheat their sentence and gain freedom, or earlier access to it.
And here's where the lowest common denominator comes in. Everything has been taken away from a prisoner. They are socially shunned, locked away, given only the barest of personal items and no sense of privacy, in a place that can be unpredictably wild and violent. In short, they don't have much to lose, and as I've said before, the most dangerous person out there is someone who feels they have nothing to lose. Inhibitions that might otherwise hold them back get stripped away.
So on that basis alone, prisoners should be banned from giving testimony to the state, though I do think that released prisoners who are not facing any charges should be able to inform the jury of what they may have heard during their time in prison. After all, they are no longer in a situation where they have nothing to lose and something to gain by violating the truth at the expense of someone else.
Korinda Martin is a particularly good example of this, because she had shown she would try anything to get out of prison, including the creation of false letters (not to mention numerous attempts to contact anyone who might be able to help her). This would not be a person I trust if she had any possibility of gain whatsoever. Her forgeries and her testimony in court showed that she obviously looked out for herself first, and would do pretty much anything to gain her freedom.
Aside from that, I also think that it is not unreasonable to examine the social atmosphere of a prison. It has, by all accounts, an extremely volatile social code based on power and prestige. And we're not talking about the community service worker kind of prestige. But, as an example, what a person has been charged with or convicted of is of great interest to prisoners, and the kind of crime goes a long way towards their initial placement in a prison's social ladder.
In such an atmosphere, I don't think it would be unlikely for a person - especially those without prior records - to express bravado about certain charges in an attempt to stave off abuse. I have heard people who have never been arrested make outrageous claims about what they would do and say "if they ever went to prison," and little of it had to do with honest self-representation. Lie or not, such statements were all about self-protection and survival. In short, the very social atmosphere is not a healthy environment for the flourishing of truth. And although I really don't think this was the case for Kirstin Lobato, I still think that there is great potential for an innocent person to lay claim to certain "power" charges in order to establish a stronger social standing (for personal survival, if nothing else). So by that, I think that what people say in prison about the allegations against them should be judged with extreme skepticism, especially when the testimony that comes from that could be unintentionally tainted by the innocent indivdual themselves. Prison society itself can act as a form of a coercion for making self-damning statements. And in a case like that, the prison informant wouldn't be lying about what they heard.
There are too many potential problems with prisoner informants such as Korinda Martin to expect reasonably accurate testimony. If the police and labs and prosecutors must go through such great pains to keep forensic evidence secure and untainted, why allow the drawing of witnesses from an atmosphere where statements are so likely to be tainted? As it did in Kirstin Lobto's case, the effect of a prisoner informant can undermine the fairness of a trial. I believe it to be an unconstitutional practice.