Twashta was one of the prajaapati created by Lord Brahma for increasing the mankind
Twashta had one son by name Viswarupa[Thrisiraska, as he had three heads].Viswarupa was a great person and was jitendriya. Viswarupa took the advice of his father and started tapasya and was doing it with great devotion .Lord Indra was jealous and was afraid that Viswarupa may take away his throne with his tapassakti.Indra wanted to disturb Viswarupa . Indra sent all the apsaras like rambha, menaka and others who made futile efforts to disturb Viswarupa.They returned to Indra and said that they could not disturb Trisiraska from his tapasya.Lord Indra got angry and he himself went to disturb Trisiraska , but he failed to do so.Then provoked by his bad thinking he cut the head of Trisiraska with his vajrayudha.Though his head was cut Trisiraska body was shining as if alive. Then Indra still was afraid that Trisiraska was alive , he asked Taksha to chop the heads of Trisiraska and bribed him with some boons if he did so .Taksha though hesitated in the begining , was attracted to the bribe offered by Indra and chopped the heads of Trisiraska .Different types of birds originated from the chopped heads of Trisiraska.
Twashta heard the end of his beloved son Trisiraska and was full of anger towards Indra and he did a Abhichaara homa and created a maasive gaint from his krodhagni and named him Vritrasura as he was born to give him relief from his duhkha(Sorrow).As Vritrasura was born he was huge and asked his father to order him what to do.Thwasta told him the story of his elder brother who was kílled by Lord Indra and asked him to kíll Indra and make him happy.
Vritrasura went to tapasya and did tapasya for hundred years for Lord Brahma. Indra wanted to disturb his tapasya also but he failed to do so.
Lord Brahma appeared and asked Vritrasura what he wanted. Vritraasura asked Brahma to give energy to defeat Lord Indra and also a boon such he cuold not be kílled in war by any wet or dry weapon .Lord Brahma gave all the boons and went back.Vritrasura went back to his father and told about his boons.
Twashta was happy and encuoraged his son to go to war against Indra who kílled his elder son.
Vritrasura defeated Lord Indra in the war and Twashta was happy , but he warned his son to be cautious abuot Indra who was cunning.
Indra who culd not defeat Vritrasura in the war and wanted to kíll Vritrasura by cheating him as a friend.Indra planned a plot with Saptarshis who went to Vritrasura and said that Indra wanted to be a friend of him.Vritrasura believed the words of the Saptarshis sent by Indra and commited to be a friend of Lord Indra. Twashta warned Vritrasura about Indra but he did not hear the words of his father.
One fine day Lord Indra with the help of Lord Vishnu kílled Vritrasura in the afternoon time with his Vajrayudha which he concealed in the froth of the ocean.
As Indra kílled Vritrasura who was the son of Twashta who was a Prajaapathi a successor Lord Brahma he acquired the sin of brahma hatya.
Though a brahmin by his birth as he was created from anger and desire to kíll, aganst Indra who is sura(devata) he became Asura(enemy of sura).