SB 6.12.21
khalv idam mahad ascaryam
yad rajah-prakrtes tava
vasudeve bhagavati
sattvatmani drdha matih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O Vrtrasura, demons are generally conducted by the mode of passion. Therefore, what a great wonder it is that although you are a demon, you have adopted the mentality of a devotee and have fixed your mind on the Supreme Personality of Godhead, Vasudeva, who is always situated in pure goodness.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
King Indra wondered how Vrtrasura could have been elevated to the position of an exalted devotee. As for Prahlada Maharaja, he was initiated by Narada Muni, and therefore it was possible for him to become a great devotee, although he was born in a family of demons. For Vrtrasura, however, Indra could not detect such causes. Therefore he was struck with wonder that Vrtrasura was such an exalted devotee that he could fix his mind without deviation upon the lotus feet of Lord Krsna, Vasudeva.