The sudden change in the way that walker moved and spoke deciding to leave them behind was odd to the light devouring dragon. However, he also realized that Walker was not stupid in any way. Isnetad, he was exceptionally insightful.
The light devouring dragon dragon did not doubt for a single section that Walker also sensed the weakness in him, potentially even better than he himself did. Therefore, Walker was acting a little colder because he knew the risk that the light devouring dragon would go through.
As much as the light devouring dragon did not want to leave the duty unfulfilled, he had no choice. The light dragonoids had also used substantial amounts of light mana. They had even used some of it to ensure that the darkness mana spreading around after one of the undead were defeated did not cause issues for him.