Agastya Rishi, who helped Sri Rama, is the one who cursed Nahusha and turned him into a Naga. This may be a ruling that said – go and live as a Naga among the Nagas. In the Valmiki Ramayana, you find characters like Tataki and Kabandha and others who were turned into Rakshasas by curses and Sri Rama turned them back into Yakshas, Gandharvas etc. If you look at curses and boons as rules and grants made by the Rishis and Brahma, which everyone had to follow, a new sense of the stories will emerge. All these accursed people waited for Sri Rama, who heard their stories, redeeemed their curses and allowed them to go back to their people.
Brahmanas are stereotypically described as satvik, in the sense of gentle or passive. But all of them were prone to anger ( a rajasic trait), and were capable of prouncing curses. Durvasa is the most famous for his temper, Viswamitra had to give it up to become a brahmarishi, but even the gentle Vasishtha got angry with king Nimi and pronounced a curse on him. It was a common belief that blessings (vara) and curses (saapa) pronounced by brahmans come true, because they are committed to truth. You do not want to see them reach for a kamandalam (water-pot), say a mantram over the water and sprinkle that water on you! Even today it is common to seek the blessings of Sat Brahmanas (good brahmans) on auspicious occasions.