Good Question on the Ethics of Calling Cops Liars
Well, in that case, I don't have to worry; this cop has lost the presumption of truthfulness by lying on previous cases. This cop is a bit of a zealot, which isn't bad by itself, but in this cop it's combined with a strong sense of "to hell with the law, this is a bad person who must be punished, so I'll say what's required to send them to prison."
But let's assume the cop hasn't got a background. If my client tells me he didn't have any drugs, the the police officer is a liar. Moreover, is the D.A. being unethical? In essence, the D.A. is calling my client a liar by filing charges/ going to trial. Considering the presumption of innocence, the D.A. would truly have an ethical problem if the same thinking were applied to the other side of our adversarial system.
However, the reason we have courts and judges and juries and lawyers and an adversarial system of justice (inefficient in time and money as it is) is because we can't know who's telling the truth, the cop or the defendant. In an adversarial system, I take up the position that my client, the defendant, is telling the truth. The D.A. stands up for his/her side of the argument; that the cop is telling the truth. Then 12 people decide who's telling the truth. Furthermore, with the presumption of innocence, the defendant doesn't have to have a story or even be telling the truth (but if I know he's not telling the truth, he's either not testifying or he's giving narrative testimony. (PD's don't get to withdraw in situations like that)) My job/duty is, at its simplest, to bring or highlight the evidence that shows the truth isn't on the D.A.'s side.
There are also some practical considerations when trying a case where there are two mutually incompatible assertions being argued. e.g., "He did it" vs. "No, he didn't", i.e., just about every case that's filed.
If you tell a jury that a cop is lying they'll shut down on you, unless you've got airtight proof. (In which case the D.A. would have dumped the case already)
If you couch the incompatible assertions as a misunderstanding or a misperception, that's easier for a jury to accept.
Finally, if you're in a situation where you've just got to flat-out say, "The cop/witness/alleged victim is lying, don't say it. Show it. Lay the problems in the evidence out for the jury and let them come up with the idea that the D.A.'s witness is a liar. This method gets around the problem of the jury tuning you out, as well as some of the ethical concerns anonymous has; all you're doing is talking about the evidence.
I don't like calling people liars. You'd better have some good evidence before I'll decide someone's a liar. But here's what I think may be anonymous' error; that requirement for good evidence before calling someone a liar? It applies to defendants as well as cops. If anonymous believed not only that cops were truthful unless proven liars, but also that citizens accused of crimes were truthful unless proven liars, he'd have the same ethical question for prosecutors.
Our system is an adversarial system, and even with all its warts, I'll take it over any other system (though I'm open to suggestions for alternatives)
And he cop wanting payback? If it's a good cop, he/she won't. If it's a bad cop, my pointing that fact out isn't going to make a bit of difference in what he/she does.
Thanks for the question, anonymous - passioned, yet polite.