I considered addressing Amon in my post too, because it's yet another layer which makes the character of Fandaniel as a whole not make sense.
He insists he sees Hermes as nothing more than a memory/soul graft, and he makes at least a couple of comments on how much the man he used to be would hate what he's doing, and all I can think is... would he really? And then you have the obvious parallels with him always being a misanthropic scientist who's got something of a beef with death and the mEaNiNg oF LiFe, and even his death monologue inside Zodiark plays the Elpis night theme.
I want to say it's just him being in denial and honestly pretty cringe about it, but I'm not entirely comfortable with that conclusion either, because reuniting the two sides as one character works awkwardly at best IMO. But then maybe it's because Hermes himself is an awkward character in the first place, because the dude seriously went from "EVERY LIFE MATTERS AND IS EQUAL (EVEN THE SPRITES)" to "yeah I just pressed the doomsday button and I know what's gonna happen lmao (but I'll pretend not to)". I honestly much preferred Fandaniel as a chaotic evil gremlin with a penchant for body horror flesh towers with doors made of teeth and sphincter walls, who angrily snaps at people around him to JUST DIE ALREADY. At least that was consistent.
Again, I have sort of managed to reason this away with my headcanon of him being originally in mental distress then slowly rehabilitated: the Sundering literally broke his self into pieces, and it just so happens that his past trauma and alienation are much more salient in his sundered mind than whatever came after. This would integrate the fact that the Leo crystal we picked up in 5.3 was bizarrely faded/altered: as we know the crystals contain only memories from the persons' tenure as Convocation members, perhaps the fact that the memories of Fandaniel were so positive and incoherent with his own experience and instincts made him reject them (as it was implied could happen with G'raha and the Exarch's memories – but of course nothing of the sort happened with G'raha, because of course not), and that broke him even more.