He was still a Nazi, who knew what was going on.
And how do you know that?
Indeed he was a Nazi, but we have no idea what he knew or suspected was going on. I doubt that very few did, it’s not like they broadcasted their intentions. That’s how Hitler got by with his secretive genocide.
Weisenthal, the great Nazi hunter, was an Austrian holocaust survivor himself and had carte blanche access to the Austrian files, journals, photographs. They take pride in their recording keeping, just like the Germans.
I’m sure they would have charged and convicted Waldheim if there was a scrap of evidence that he committed a war crime. It would have been one of the biggest successes of the tribunal.
FWIW, The New York Times tried to smear / convict Waldheim to kill his political career, but they got nowhere.