Zacharias Moussaoui’s death penalty case is a fascinating interesting brain twister.

Moussaoui claims that he was in an al Qaeda plot to use hijacked airliners to attack American targets — just not the 9/11 plot.

The government counters that he was in the 9/11 plot, which fact, it is urged, is shown most clearly by his lies. Had Moussaoui not lied when he was arrested a few weeks before the attacks, the prosecution argues, the 9/11 plot might have been revealed and stopped. That may be, but it doesn’t necessarily get the govs where they need to go in order to justify capital punishment.

That’s because, for the death penalty, it’s not enough to show that he was connected to the al Qaeda terrorists — it has to be shown that he was connected to the 9/11 plot. One could, for example, accept as a premise that truthful information from Moussaoui might have led to the capture of some of the 9/11 hijackers — but, while that might have thwarted the plot, it wouldn’t necessarily show that Moussaoui had enough knowledge of the plot to have been a member of it.

It’s also very hard to prove a positive by a negative. The government is spending lots of time showing Moussaoui is a liar and that al Qaeda guys lie as a matter of course. But you can’t establish the truth of an allegation merely by showing the defendant lied.

An example: If the bank gets robbed on Tuesday and I falsely tell the police I can’t have robbed the bank because I was at the ballgame, proving I was not at the ballgame shows that I’m a liar, but it doesn’t make me a bank-robber. The government would need some positive proof that I robbed the bank. If it had such positive proof, my lies would be very powerful confirmatory evidence of guilt (called “false exculpatory statements” in trial lawyer lingo). But by themselves, my lies would be smoke without fire.

And then there’s the strange dynamic of Moussaoui and the Judge. He is proud to be a member of al Qaeda and gets smiley when things like the Cole attack come up. It can’t make a jury of Americans very happy to see a terrorist doing the Icky-shuffle over a successful strike. So the judge is making considerable efforts to rein in his unruly behavior.

But while the antics are offensive to courtroom decorum, why shouldn’t the jury get to see exactly who this guy is and where he’s coming from. And it’s hard to view the jury’s witnessing these hijinks as “unfairly prejudicial” to Moussaoui (i.e., as something that might cause jurors to vote for death in the absence of sufficient evidence). Moussaoui himself is intentionally causing the disruptions with his unbridled glee over dead Americans. To bring it full circle, that unbridled glee is an exacerbating factor that it’s appropriate for the jury to weigh in deciding whether he should be executed … but only if the government can preliminarily establish the complicit link between Moussaoui and the 9/11 plot.