As I recall at the time the Cheney-Wolfowitz-Rumsfeld cabal did a smowjob on him and convinced Powell that the case for WMDs was airtight. He was livid when it came out later that the whole yellowcake uranium story was a fabrication. He felt they'd destroyed his hard-bought reputation for honesty to advance their political ends. They'd used him.
The State Department, whom Powell had been head of, had been against the war based on the intelligence that it's intelligence arm, the Bureau of Intelligence and Research (INR), had had, they were about the only agency that resisted pressure from the Bush/Cheney administration to provide "evidence" favorable to a cause for war. Up to the time that Powell let himself be put up to present the Administration's case before the UN, the INR provided assessments that the info he was about to deliver was bogus, for various reasons such as questionable sources, outright lack of evidence and contradictions, and overall lack of customary vetting.
He wanted so hard to be a good soldier to the President that he allowed himself to believe in the bogus evidence all the while he had had evidence that countered it. His is a textbook example of cognitive dissonance.