SB 6.18.20

śrī-rājovāca
kathaṁ ta āsuraṁ bhāvam
 apohyautpattikaṁ guro
indreṇa prāpitāḥ sātmyaṁ
 kiṁ tat sādhu kṛtaṁ hi taiḥ
Synonyms: 
śrī-rājā uvāca — King Parīkṣit said; katham — why; te — they; āsuram — demoniac; bhāvam — mentality; apohya — giving up; autpattikam — due to birth; guro — my dear lord; indreṇa — by Indra; prāpitāḥ — were converted; sa-ātmyam — to demigods; kim — whether; tat — therefore; sādhu — pious activities; kṛtam — performed; hi — indeed; taiḥ — by them.
Translation: 
King Parīkṣit inquired: My dear lord, due to their birth, the forty-nine Maruts must have been obsessed with a demoniac mentality. Why did Indra, the King of heaven, convert them into demigods? Did they perform any rituals or pious activities?