I remember the lead up to the Iraq war. And I remember listening very closely to how Powell talked vs Bush & Rumsfeld. Powell appeared to be doing everything he could, short of rebellion, to stop the war from going ahead. His words were incredibly considered - alluding to moderate courses of action while not outright disagreeing with his President. I wasn't surprised at all when he was eventually removed.
This of course, is my opinion. And it is to say - I doubt he believed in WMD's at all. Or in the war.