It wasn't just what the Daeva did; it was also what came before, and what it represented. Odyssial's people had built a large and prosperous city-state, through long struggle and hard work, persevering and flourishing in a hostile world. Then out of nowhere a Primordial destroys it and wipes out almost the entire population for no greater reason than that he felt like like sitting down in that particular spot and neither noticed nor cared about the thriving civilization he was destroying.
Then Odyssial becomes the leader of the remnants of his shattered people; he's the greatest a mortal can be, and he employs every single trick a mortal can possibly employ to keep them alive, and it's still not enough; all because the Primordials deliberately made man weak, so weak that even the greatest of them couldn't hope to survive without aid, for the sake of collecting more prayer from them.
And then, simply so he wouldn't lose out on some minor wager he made about their survival, a Daeva decides to slaughter all that remain. And the worst part? He doesn't even have the decency to feel anything other than perhaps a vague amusement about the whole thing.
In other words, Odyssial hated the Primordials because they created them, made them the weakest peoples of Creation, rendered almost everything in the world dangerous to them so they would be totally dependent on their creators to survive, all to generate more prayer... and then don't even have the common courtesy to actually hold up their end of the bargain and protect them.
So yeah, I can totally see Odyssial spending countless centuries building an earthly paradise that makes even gods in Yu-Shan jealous. He wanted revenge on the Primordials for what they did; he got it. They're either dead and howling in torment eternal, or mutilated and stuffed into the inside-out thing that was once their king. So with that done, what's he going to want to do?
Prove he's better than them. Build a better world for humanity and every other mortal in Creation than the Primordials or even the gods could ever dream of. Build them up, elevate them beyond what the dictates of their creators said they could be; make sure no high-and-mighty bastard can ever casually wipe out entire civilizations over a bet.
And, though he probably doesn't admit it to himself, to try to make all their deaths somehow worth it. Nio saved his life twice, and died so that he could live. They all died, and now he is the only thing that remains of a great people who once were, in a time likely forgotten even by many of his fellow Exalted. In Odyssial's mind (whether he realizes it or not), everything he does is probably measured in terms of "is this the best way to use the life that Nio saved?"