I first read it that he deliberately tuned Angeal out when he shut his eyes and jumped, and that's why Angeal looked horrified: because Seph had just voluntarily turned his back on the last bit of humanity. (;_;)
But then I reread it and thought maybe Seph closing his eyes was a gesture of faith instead of dismissal, and then once he'd jumped Angeal discovered he had just accidentally lured him straight into Jenova's trap. So he'd been trying to reach for Angeal's faith in humanity, and Angeal's guidance failed him. (also ;_; in very different directions)