Curate, connect, and discover
Diavolo's plan seems hugely dependant on all three races as a whole being individually under one definitive ruler, (Michael, The Demon King) with this being undoubtedly and obviously false for humans I really just have to wonder if he even truly understands that fact.
He knows compared to demons and angels there are a small amount of humans, but does he account for how widespread and disconnected from each other we are?