Emet-Selch was tempered. He couldn't really go against Zodiark if he wanted to. His goal was to win you over to become his friend, witness the futility of lesser beings so that you could transcend above them. If you absorbed all that light and brought it back to the source yourself then it wouldn't have been much different than the Ascian way.

Basically you are a part of the source and if absorbing other warriors of light to become the original you helps you hold all the light then it's the same goal with a twist.

But ultimately you failed to live up to his expectations, being too little of your former self, the rejoining of souls didn't happen and you were basically going to die for it. So he concocted one final scheme to give you a chance to live.

He laid all his cards on the table in a last ditch effort to make you remember and to want in his goal so that you may live.

When you kill Hades it's still a small victory for him. You're well on your way to becoming whole again.