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. Vasistha, however, did just the opposite

Expressions researched:
"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"

Srimad-Bhagavatam

SB Canto 9

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.

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.