When "W" chose Colin Powell as Secretary of State, did he tell him that when Powell "disagreed" with the President it was his place to disregard the President's orders and then have Powell tell the UN what he believed?
Powell tried to change the President's mind but Bush was "The Decider" and what Cheney told him to do would ultimately be his decision. Cheney wanted war. Bush wanted to outdo his father so he went right along with the hairbrained plan.
Powell was given orders and, like a good soldier, he followed those orders. Yes, Powell could have resigned instead of following the orders but, elsewise, he did what he was told to do.
You are remiss if you attribute Powell's speech to HIS beliefs in my opinion. He took the Intelligence (such as it was) and fashioned a presentation around it. He wasn't necessarily "against" the war. He was only against the way it was conducted. He wanted a "coalition" force with more boots on the ground and not a cowboy "shoot em up" like in the movies.
Peace