I found the Pandemonium thing to be - Zidane has always known who he was, and then Garland basically pulled the rug out from under him. He took Zidane's defining quote - you don't need a reason to help people - and basically told him that that was failing at his intended design and he was meant to be like Kuja. I can see that messing with someone who might already have been teetering after realizing he is, in some ways, the same as the person who created the black mages and controlled Brahne.