Hussein murdered how many hundreds of thousands? Did he give any of them a trial?
I don't see why he should have been given one.
In the civilized world, all men, regardless of likelihood of guilt, are presumed innocent of any crime until proven guilty of at least one beyond a reasonable doubt in a court of law. This ensures that we can have as much confidence as possible that the accused did indeed carry out the crime(s) in question and will not be subject to punishment for something that he or she did not do.
I don't see why exceptions should be made for those who don't follow the same set of rules. Aren't we supposed to be better than them? The entire point of a trial is to firmly and concretely establish the guilt that you mentioned above. If it had turned out by some weird chance that Saddam had not, in fact, done any of what everyone thought he had done whatsoever, it would have been a travesty to execute him purely to act tough.
I should also point out that murderers in general never give their victims a trial. The fact that Saddam Hussein committed rather more murders than the average murderer does not change this fact. Thus, by the logic above, we should never give anyone suspected of murder a trial at all and simply execute them immediately.