harer dadhīces tapasā ca tejitaḥ
tenaiva śatruṁ jahi viṣṇu-yantrito
yato harir vijayaḥ śrīr guṇās tataḥ
Vṛtrāsura not only assured King Indra that the thunderbolt was invincible, but also encouraged Indra to use it against him as soon as possible. Vṛtrāsura was eager to die with the stroke of the thunderbolt sent by Lord Viṣṇu so that he could immediately return home, back to Godhead. By hurling the thunderbolt, Indra would gain victory and enjoy the heavenly planets, remaining in the material world for repeated birth and death. Indra wanted to gain victory over Vṛtrāsura and thereby become happy, but that would not at all be happiness. The heavenly planets are just below Brahmaloka, but as stated by the Supreme Lord, Kṛṣṇa, ābrahma-bhuvanāl lokāḥ punar āvartino ’rjuna: even if one achieves Brahmaloka, he must still fall to the lower planetary systems again and again. However, if one goes back to Godhead, he never returns to this material world. By killing Vṛtrāsura, Indra would not actually gain; he would remain in the material world. Vṛtrāsura, however, would go to the spiritual world. Therefore victory was destined for Vṛtrāsura, not for Indra.