Firstly, You've never been a conservative.
Only liberals refer to the doctrine of preemptive war as "The Bush Doctrine." The Bush Doctrine is, as Charles Krauthammer will tell you, something else entirely.
The Bush Doctrine entails the spread of Democracy around the world. Learn it, live it, love it.
As far as preemption is concerned, an invasion of Nazi Germany to liberate Jews prior to Hitler's declaration of war on the United States would have been entirely appropriate within every Christian construct imaginable.
Saddam had already gassed Kurds and was firing on U.S. planes enforcing the No-Fly-Zone. Your ignorance is sky-high.
However, you are saying that we should bomb a country because of things that Saddam had already done. Because he killed numerous people in the past, we should bomb numerous people, now, to protect them. However, by the time we bombed Iraq, the world was watching Saddam closely, and he would not have been gassing any more Kurds. There is the difference. The world is a vastly different place, now. There's not much that Saddam could have done without our notice.