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.