Firstoff, I have major doubts that executing Saddam is going to stop the killing. Once a leader of any sort is executed - he becomes a martyr, which has huge symbolic value for his followers. I belive this is why Gandhi was never brought up on capital charges when he was jailed by the British (althouhg they could have - for treason). I think Nelson Mandela was another similar case... although I cant be sure. Of course, Saddam can't be compared to those guys, but in the eyes of his followers, he probably can.
But anyway, the question of whether or not to execute him should be based solely on what he did, not what people who view him in a certain way might do at some point in the future.
If on the other hand, they ARE going to consider the effects of the sentence on his followers, they're better off putting him in prison and periodically releasing footage of him watching ESPN and eating cheeseburgers.