PURPORT

In this connection Srila Visvanatha Cakravarti Thakura comments:

brahmesendradaya iti. indrasya sva-dhisnya-gamanam nopapadyate
vrtra-vadha-ksana eva brahma-hatyopadrava-prapteh. tasmat tata ity
anena manasa-sarovarad agatya pravartitad asvamedhat parata iti
vyakhyeyam.

Lord Brahma, Lord Siva and the other demigods returned to their respective abodes, but Indra did not, for he was disturbed at having killed Vrtrasura, who was actually a brahmana. After killing Vrtrasura, Indra went to the Manasa-sarovara Lake to become free from sinful reactions. When he left the lake, he performed an asvamedha-yajna and then returned to his own abode.