Religion

He probably had more of an inroads with the Facists and Nazis than he thought. Italy being mainly Catholic and Hitler (through his mother) and parts of Germany significantly Catholic. That he didn't push this relationship to actually change what happened to Jews and any other Nazi opponents in Europe is the big problem in my mind.

It seems he was more interested in maintaining the Church (its peoples and properties) than arguing for respect for all people, including people of other (or no) faiths. And I guess that's the larger tragedy of all of this, he could have (given his potential religious relationship with both Hitler and Mussolini) and should have done more.