Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 6
वृत्रस्तु स कथं पाप: सर्वलोकोपतापन: ।इत्थं दृढमति: कृष्ण आसीत्सङ्ग्राम उल्बणे ॥ ६ ॥
vṛtras tu sa kathaṁ pāpaḥsarva-lokopatāpanaḥitthaṁ dṛḍha-matiḥ kṛṣṇaāsīt saṅgrāma ulbaṇe
Vṛtrāsura was situated in the blazing fire of battle and was an infamous, sinful demon, always engaged in giving troubles and anxieties to others. How could such a demon become so greatly Kṛṣṇa conscious?
It has been described that a nārāyaṇa-parāyaṇa, a pure devotee, is rarely found even among millions and millions of persons. Therefore Parīkṣit Mahārāja was surprised that Vṛtrāsura, whose purpose was to give trouble and anxiety to others, was one of these devotees, even on a battlefield. What was the reason for Vṛtrāsura’s advancement?