Hubris. He was the strongest and thought he was certain to win the prize. He'd always been a psycho, but being a step away from having even more power sent him off the deep end. His conversation with the priest about God's "undoing" shows where his ego is at.
His "fade away" line at the church shows he's already unconcerned with dying, and when he's beheaded the look on his face is not anger/despair at losing, but instead he looks relaxed and even starts laughing, like saying "well, whatever, it's finally over anyway".
Or am I reading too much into it? Don't think the "he's so arrogant he thought he could survive the beheading" explanation would make sense. Otherwise why bother defending himself at all?
I got the same impression when he died. It was a look that told me he was glad it was finally over, regardless of the winner. Probably the closest thing to a redeeming moment the character has, and I like imagining that as his last thought. So I choose to interpret it that way.