Srimad Bhagavatam Canto 06, Chapter 14, Text 07

SB 6.14.7

atra nah samsayo bhuyañ
 chrotum kautuhalam prabho
yah paurusena samare
 sahasraksam atosayat
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
My dear lord, Sukadeva Gosvami, although Vrtrasura was a sinful demon, he showed the prowess of a most exalted ksatriya and satisfied Lord Indra in battle. How could such a demon be a great devotee of Lord Krsna? These contradictions have caused me great doubt, and they have made me eager to hear of this from you.
Srimad Bhagavatam Canto 06, Chapter 14, Text 06
Srimad Bhagavatam Canto 06, Chapter 14, Text 08