I don't think that all evil acts must be accommodated by a criminal prosecution. Crime implies punishment, deterrence, and some sort of due process. I think that sometimes evil people go beyond crime, that treating them as criminals cheapens the horror of their evil and lends them some regard and dignity that is not and should not be their due.
Now that Winston Churchill's notebooks are being released to the public, we are seeing that he was not a bit squeamish about the idea of destroying his enemies. His thought was that the Nazi top brass should have been shot at the first opportunity by anybody readily available, rather than having been tried as war criminals. Dymphna at the Gates of Vienna has it exactly right. Taking off from Churchill's attitude about Hitler, that the war crimes trials are only good if a defendant grovels for his crimes, Saddam shouldn't be given a trial where he is treated with any respect or dignity. If he isn't willing to grovel at his trial, shoot him.
But if you have to have a trial, to my mind, a better model to follow for Saddam's treatment after capture would have been that of Ceausescu. That is, run through a quick and private trial and then impose the final sentence immediately without appeal and dump the trash secretly so no morons try to build a monument on the gravesite. (To paraphrase, the moronic are always with us.)
Certainly, the idea of a trial with the routines of due process seems a bit much in the case of bloody-minded people like Saddam . The attempt to provide a pious and public display of due process inevitably is turning into a collectivist-type show trial, which merely gives Saddam additional opportunity to act out the alternate parts of Islamic strong man or emblematic victim of western persecution of the ummah.
Give him enough due process and he'll become the new caliph, by and by.