Ignis can't blame Prompto for the anger he's feeling. Ever since he found out in Altissia what kind of an end is waiting for their King, he felt as if his hands were tied. And, when he couldn't dissuade Noct from his intention of carrying on with the task, the lock has been put where the chains were on his hands. There was nothing they could do or say to change Noct's mind.

And when it came down to the Crystal- it was both their savior and doom. No, it wasn't enough. And Noct wasn't enough either. It took generations of Lucis Caelum to banish the darkness. If the Prophecy is right.

"The Ancestors that doomed the World- Noct tried to give us what little peace he could. We must cherish it and never lose the sight of what was given to us." He's trying, desperately, not to give into the anger he's feeling himself. Because he's sure that's something Noct wouldn't want them to do.