Wood Spirit Tribe's Allegiance
After Vaan and Elder Willowthorn met up, all of Elder Willowthorn's doubts were dispelled with Vaan's brief explanation.
Elder Willowthorn learned that the disappearance of the death energy and Death Eaters was indeed related to Vaan. He quickly expressed his relief when he found out the Death Eaters were simply moved to another subspace and not annihilated.
Although the Death Eaters were lifeforms of opposing nature to the Wood Spirits, they were still a type of lifeform.
As such, Elder Willowthorn thought it was too cruel to eliminate the Death Eaters after they had served their purpose of guarding the inheritance.
His concern seemed universal and did not discriminate despite his incompatible attributes with Death Eaters.
Such was the nature of Wood Spirits.
The Wood Spirit Race sought harmonious co-existence with all beings and wouldn't intentionally harm others for self-interest. Wood Spirits would rather be hurt than hurt others.