It is not seriously disputed that had Saddam attained his goal of acquiring nuclear weapons he would have used them. That, in and of itself, may not, in the minds of some, constitute justification for the invasion into his land, but as the author of more than a million deaths in two wars already, not to mention the deaths of thousands of his own people by poison gas, his taste for mass murder could not be doubted.
Morover, there was ample reason to believe that he had stockpiles of chemical and biological weapons, not least being the considerable quantities of these that were known to exist but were, at the start of the war, unaccounted for. So the argument over whether there was sufficient justification for military action to depose him, or the moral basis for it, boiled down to this: Whether it was better to act to neutralize the threat he posed before it came to fruition, or wait until he actually possessed such weapons before responding.
Bush made the right move.