The Shatpatha Brahmana states that Vritra was created by Tvastar to avenge the killing of Trisiras by Indra. However, many verses in RV call Tvastar the creator of Vajra for example Rigved 1.32.1 says:
1 I WILL declare the manly deeds of Indra, the first that he achieved, the Thunder-wielder. He slew the Dragon, then disclosed the waters, and cleft the channels of the mountain torrents. 2 He slew the Dragon lying on the mountain: his heavenly bolt of thunder Tvaṣṭar fashioned. Like lowing kine in rapid flow descending the waters glided downward to the ocean.
My question is, why did Tvastar, create the only weapon that could destroy his own son?