I agree with you completely. I suppose most liberals think assassinating even a bastard like Saddam is wrong because there's no trial. Also, they can't bear the idea of "collateral damage", even if the number of innocents who die in an assassination attempt is far lower than in a war.
But will killing Saddam and his entourage be enough to cause the downfall of the entire Iraqi regime?
Most leaders would think twice about waging war if they were the targets of the war. I think assassination and other means of targeting leaders should become legitimate tactics of war.
But will killing Saddam and his entourage be enough to cause the downfall of the entire Iraqi regime? The next leader would know that if he did not behave, he would be the next target, and being a constant target spoils the fun of being a dictator.