He made stupid decisions, yes, but it was established that he had no conscious intentions of hurting anyone or destroying the world throughout all of Elpis until the end. He was self-centered, he was contemptuous of others, he was naive. In that moment, hearing Meteion's conclusion and Emet's words striking a bad nerve in him, yes, it was horrifying and despicable, but he did not intend to do it until that moment. There was no intent until after he fought the party, listened to the full report, and decided he really wanted to spite Emet-Selch by turning his own words (and the ideals of the society he loved) against him.

I am hard on board the reading that Amon = Hermes in all the ways that are important, and his protests otherwise are mostly self-delusion, but even Amon says - and I think we're supposed to take him at his word - that the Hermes of the past would hate what Amon actually became.