Why bin Laden was radicalized

I spent three days with Osama bin Laden in 1996 when I traveled to the remote, mountainous Tora Bora area of Afghanistan to interview him. I found him gently spoken, humble -- even shy -- and extremely polite. It is difficult to reconcile these personal memories of the man with the fact that he presided over so much terror and destruction. Why did he hate the Americans so much? Why did he turn to the most extreme form of radical Islam and why did he chose violence to express this hatred and this faith?