Srimad Bhagavatam Canto 06, Chapter 18, Text 43

SB 6.18.43

pratisrutam dadamiti
 vacas tan na mrsa bhavet
vadham narhati cendro ’pi
 tatredam upakalpate
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
I promised to give her a benediction, and this promise cannot be violated, but Indra does not deserve to be killed. In these circumstances, the solution I have is quite suitable.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
Kasyapa Muni concluded, “Diti is eager to have a son who can kill Indra, since she is a woman, after all, and is not very intelligent. I shall train her in such a way that instead of always thinking of how to kill Indra, she will become a Vaisnava, a devotee of Krsna. If she agrees to follow the rules and regulations of the Vaisnava principles, the unclean core of her heart will certainly be cleansed.” Ceto-darpana-marjanam. This is the process of devotional service. Anyone can be purified by following the principles of devotional service in Krsna consciousness, for Krsna consciousness is so powerful that it can purify even the dirtiest class of men and transform them into the topmost Vaisnavas. Sri Caitanya Mahaprabhu’s movement aims at this purpose. Narottama dasa Thakura says:
vrajendra-nandana yei, saci-suta haila sei,
 balarama ha-ila nitai
dina-hina yata chila, hari-name uddharila,
 ta’ra saksi jagai-madhai
The appearance of Sri Caitanya Mahaprabhu in this Kali-yuga is especially meant to deliver the fallen souls, who are always planning something for material enjoyment. He gave the people of this age the advantage of being able to chant the Hare Krsna mantra and thus become fully pure, free from all material contamination. Once one becomes a pure Vaisnava, he transcends all material conceptions of life. Thus Kasyapa Muni tried to transform his wife into a Vaisnavi so that she might give up the idea of killing Indra. He wanted both her and her sons to be purified so that they would be fit to become pure Vaisnavas. Of course, sometimes a practitioner deviates from the Vaisnava principles, and there is a chance that he may fall down, but Kasyapa Muni thought that even if one falls while practicing the Vaisnava principles, he is still not a loser. Even a fallen Vaisnava is eligible for better results, as confirmed in Bhagavad-gita. Svalpam apy asya dharmasya trayate mahato bhayat: even practicing the Vaisnava principles to a small extent can save one from the greatest danger of material existence. Thus Kasyapa Muni planned to instruct his wife Diti to become a Vaisnava because he wanted to save the life of Indra.
Srimad Bhagavatam Canto 06, Chapter 18, Text 42
Srimad Bhagavatam Canto 06, Chapter 18, Text 44