It's the Cambodian story that does it. At the end of that story line, Nate's faith in humanity and their ability to make their own decisions is basically gone. At that point, which if I remember right was right about when Armageddon 2001 was going on, it made perfect sense. Nate hits a complete low, no longer believes that humans can make their own decisions, starts interfering more and more, the JLA and others set out to stop him, he fights back, becomes Monarch and begins wiping out the rest of the heroes so they can't stop him from directing humanity.