OVERANALYZING THE TEXT

I agree with Barb, when she writes:

Indeed, we have yet to see a really extensive depiction of wizarding justice. We saw what we thought was a miscarriage of justice when Crouch sent his son to Azkaban (turned out to be perfectly right) and what seemed to be justice when Bagman was released (there are hints from Winky that he is far more sinister than he appeared, and the twins' opinions notwithstanding, possibly brighter than he seemed as well). We also see an episode that is common in the Muggle world: someone making a deal to get released, with no intimation that they were not completely in the wrong (Karkaroff naming names).

Yes. I'd also like to add that while people keep referring to these scenes as "trials," they aren't really. What we see of Crouch and Bagman's trials are only the sentencings. Both parties would seem to have been already found guilty in their respective scenes (Bagman was not acquitted of the charges against him; he was merely absolved from penalty, which is not at all the same thing). What we see in both cases is the declaration of verdict and the sentencing, not the trial as a whole.

And of course Karkaroff's scene isn't a trial at all. It's some kind of hearing, but it's not exactly a trial.

We have no idea what evidence may have been presented over the course of Bagman or Crouch Jr.'s trials, nor what the rules of preponderance of evidence might be in the wizarding world.

Sirius claims that Crouch the Lesser's trial was little more than a show-trial, but his knowledge of the event must be second-hand—he was in prison at the time. From Dumbledore's comments on the unreliability of the Longbottoms as witnesses, and from his admission that he was not absolutely convinced of young Barty's guilt, we might infer that Dumbledore, too, feels that the evidence was scanty and the outcome possibly unjust. And it's certainly obvious that an ugly mood prevailed over the proceedings. The trial was certainly biased.

But we can't really say that it was improper. It's possible that the rules of preponderance of the evidence are just not very strict in the wizarding legal system. It's also possible that the onus of proof within the system falls upon the defendant—that it's a "guilty until proven innocent" system. While both trials do strike us as rather dubious according to the Spirit of Justice, they may well have been perfectly within bounds of the legal system itself.

However...

The only other "justice" we know of is Sirius' being imprisoned without a trial--but it seemed that his deep-seated feelings of guilt for switching the Secret Keeper were as much to blame for this, as we never hear of him demanding a trial. He seems to have gone off willingly (he's said to be laughing madly).

However, I don't know about this. It's possible, I suppose, that by the laws of the wizarding world, the defendant only gets a trial if he wants one, or that confession obviates the need for a day in court...but I don't quite believe it. Sirius certainly seems bitter enough in retrospect about having been sent to prison without trial, and he speaks of it as if it were an extraordinary event: an exception to normal legal proceedings, rather than an unfortunate by-product of his state of mind at the time of his arrest.

Although if wizarding law did hold that a criminal who confesses his guilt does not need to stand trial, that would cast the Shrieking Shack scene of PoA in a somewhat different light, wouldn't it?

—Elkins