PURPORT

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. 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. Therefore Srila Visvanatha Cakravarti Thakura says that he was durmati; in other words, his intelligence was not very good.