Kasyapa Muni said: If you (Diti) deviate from this vow of following the Vaisnava principles, you will get a son who will be favorable to Indra
SB Canto 6
Kaśyapa Muni said: My dear gentle wife, if you follow my instructions regarding this vow for at least one year, you will surely get a son who will be able to kill Indra. However, if you deviate from this vow of following the Vaiṣṇava principles, you will get a son who will be favorable to Indra.
The word indra-hā refers to an asura who is always eager to kill Indra. An enemy of Indra is naturally a friend to the asuras, but the word indra-hā also refers to one who follows Indra or who is obedient to him. When one becomes a devotee of Indra, he is certainly a friend to the demigods. Thus the words indra-hādeva-bāndhavaḥ are equivocal, for they say, "Your son will kill Indra, but he will be very friendly to the demigods." If a person actually became a friend of the demigods, he certainly would not be able to kill Indra.