The ever-eloquent Anonymoose has done a pretty excellent job detailing everything, but I'll still give my own interpretation on events if only to sate my ego.
Emet-Selch is a man driven far more by his emotionality than efficiency in scheming. He's very good at that too, but part of the whole bait and switch of the character is that he initially appears like some megalomaniacal madman but turns out to be a bitter, extremely depressed survivor filled with loneliness and guilt. He, far moreso than even other unsundered we've seen, truly cannot escape the past and obsessively dreams back towards it. He can't let go of the glory of Amaurot to the point where he creates a massive replica of it, even admitting it's far more than would be necessary for his goals. For him, everything goes back to the final days and the convocation's reaction to them. He sees even his grand nation-building projects as little more than playing around with toys since it's utterly impossible for any of them to match to Amaurot.
He's also kind of a racist, believing the unsundered to be little more than crude monkeys incapable of real emotion, civilization, or accomplishment. This relates back to the fascist themes of the overall game, especially denoted since the civilization he's most linked with, Garlemald, has clear and evident fascist themes to it. The unsundered are toys, even if he's tried to relate to them he ultimately failed. (As a slight digression I interpret that failure as more of his guilt and obsession with the Unsundered at play rather than any truth. The game makes it abundantly clear that sundered mortal life possesses inherent value and the capacity for greatness.) But at the same time, he cares about one sundered mortal...namely, the Warrior of Light.
Why?
Because the Warrior of Light presents a chance to rewrite history. Azem was pretty explicitly Emet-Selch's best friend, and the one the ascian most respected. But despite that, when Emet-Selch and the others settled on the Zodiark plan, Azem disagreed. And for a man as obsessed with the past like Emet-Selch, that fact tore him up inside for millennia. But here's a chance, maybe a small chance, and he tries not to hold too much hope on it, but maybe this time he can get Azem to agree? If he gets the Scions on his side they can figure out a way to handle the rejoining without killing the WoL or them turning into a Sin-Eater. He'll have time and some of the best-equipped mortals to help him too, which is nice, if not totally necessary. But more than that he can finally prove that he was right all of those thousands of years ago. He can make Azem see things his way, and if it doesn't work...well, he'll just kill them and move back on with the plan. However, just as he hoped that the Warrior of Light might truly be Azem...they prove to be just another disappointing mortal. But he held onto some hope, maybe if he reminds them just right he can see them as a real human again...and if not he'll just kill them. No big deal.
Unfortunately for Emet-Selch, he can only overcome his faults in his last moments. Only then does he actually trust the mortals to inherit the legacy of Amaurot. Only then does he finally see the WoL as truly similar to Azem. Only then, does he find peace.