It appears that Vasistha was not free from tamo-guna, the mode of ignorance. As the family priest or spiritual master of Prsadhra, Vasistha should have taken Prsadhra's offense very lightly, but instead Vasistha cursed him to become a sudra

Expressions researched:
"It appears that Vasiṣṭha was not free from tamo-guṇa, the mode of ignorance. As the family priest or spiritual master of Pṛṣadhra, Vasiṣṭha should have taken Pṛṣadhra's offense very lightly, but instead Vasiṣṭha cursed him to become a śūdra"

Srimad-Bhagavatam

SB Canto 9

It appears that Vasiṣṭha was not free from tamo-guṇa, the mode of ignorance. As the family priest or spiritual master of Pṛṣadhra, Vasiṣṭha should have taken Pṛṣadhra's offense very lightly, but instead Vasiṣṭha cursed him to become a śūdra.

SB 9.2.9, Translation and Purport

Although Pṛṣadhra had committed the sin unknowingly, his family priest, Vasiṣṭha, cursed him, saying, "In your next life you shall not be able to become a kṣatriya. Instead, you shall take birth as a śūdra because of killing the cow."

It appears that Vasiṣṭha was not free from tamo-guṇa, the mode of ignorance. As the family priest or spiritual master of Pṛṣadhra, Vasiṣṭha should have taken Pṛṣadhra's offense very lightly, but instead Vasiṣṭha cursed him to become a śūdra. It is the duty of a family priest not to curse a disciple but to give him relief through the performance of some sort of atonement. Vasiṣṭha, however, did just the opposite. Therefore Śrīla Viśvanātha Cakravartī Ṭhākura says that he was durmati; in other words, his intelligence was not very good.