daityā devānugāḥ svayam
In this connection Śrīla Viśvanātha Cakravartī Ṭhākura comments:
brahmeśendrādaya iti; indrasya sva-dhiṣṇya-gamanaṁ nopapadyate vṛtra-vadha-kṣaṇa eva brahma-hatyopadrava-prāpteḥ; tasmāt tata ity anena mānasa-sarovarād āgatya pravartitād aśvamedhāt parata iti vyākhyeyam.
Lord Brahmā, Lord Śiva and the other demigods returned to their respective abodes, but Indra did not, for he was disturbed at having killed Vṛtrāsura, who was actually a brāhmaṇa. After killing Vṛtrāsura, Indra went to the Mānasa-sarovara Lake to become free from sinful reactions. When he left the lake, he performed an aśvamedha-yajña and then returned to his own abode.