SB 6.14.1
sri-pariksid uvaca
rajas-tamah-svabhavasya
brahman vrtrasya papmanah
narayane bhagavati
katham asid drdha matih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
King Pariksit inquired from Sukadeva Gosvami: O learned brahmana, demons are generally sinful, being obsessed with the modes of passion and ignorance. How, then, could Vrtrasura have attained such exalted love for the Supreme Personality of Godhead, Narayana?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In this material world, everyone is obsessed with the modes of passion and ignorance. However, unless one conquers these modes and comes to the platform of goodness, there is no chance of one’s becoming a pure devotee. This is confirmed by Lord Krsna Himself in Bhagavad-gita (7.28):
yesam tv anta-gatam papam
jananam punya-karmanam
te dvandva-moha-nirmukta
bhajante mam drdha-vratah
“Persons who have acted piously in previous lives and in this life, whose sinful actions are completely eradicated and who are freed from the duality of delusion, engage themselves in My service with determination.” Since Vrtrasura was among the demons, Maharaja Pariksit wondered how it was possible for him to have become such an exalted devotee.