I'm going to agree with 'Somni on the fact that the Administration really needed to emphasize the
other twenty-odd points in the
Authorization for Use of Military Force Against Iraq. Mentioning one (which happened to be the one we haven't found good evidence to support, regardless of the beliefs of literally
the entire world's intelligence community before the war) over and over and over again, while ignoring all the others, really hurt support for military action.
As for why we didn't assassinate Saddam, that's easily answered -- a few years ago (around Carter, I think), the President at the time made an Executive Order which prevents Americans and American assets, in particular the CIA, who was usually responsible for such things, from engaging in or taking part in assassinations. Certainly, another President could easily countermand the order and permit the CIA to engage in assassinations again, but who wants to be remembered as the President who reauthorized assassinations? (And the media would tear the President apart for it during their whole time in office.) Of course, you can't just blow the heck out of a country and walk away, much as we'd like to. Nation building isn't fun, but it's the responsible thing to do. And we're much better at it. It took over a decade before we let Japan govern itself again. And there was a sizeable number of people (pretty much anybody who'd seen Imperial Japan in action) who wouldn't be convinced until it happened that Japan could join the rest of the world as a functioning, contributing democracy. From what I've heard, the Iraqis are doing pretty darn well for themselves.
Of course, I've supported both of these wars from the start and don't intend to stop. *wry grin*