Shame, relation and revenge.

Cooperation between Demons and Humans, who would think it possible? The Human nature strongly goes against it no matter what religion one follows. The guilty feeling of such cooperation has grown so much in people's subconscious that it almost feels like a seventh sense.

Regardless of that uncomfortable fact, the King is working together with Demons even though at first they've approached as Hostile. We can consider desperation as the base of this teamwork, King Michael required and still needs help fighting off the clawed Demons.

It was either tolerate some low IQ bullshit threatening and find a way of working together or take his chances with the Clawed Demons. He knows that taking his chances with the clawed Demons isn't the best idea especially since the population has plunged 100 people less. Gaster Kingdom only has 700 Civilians now.