One verse

Srimad Bhagavatam 6.14.6

6.14.6

वृत्रस्तु स कथं पाप: सर्वलोकोपतापन: । इत्थं द‍ृढमति: कृष्ण आसीत्सङ्ग्राम उल्बणे ॥ ६ ॥

vrtras tu sa katham papah sarva-lokopatapanah ittham drdha-matih krsna asit sangrama ulbane

vrtrah
Vrtrasura
tu
but
sah
he
katham
how
papah
although sinful (getting the body of a demon)
sarva-loka
of all the three worlds
upatapanah
the cause of suffering
ittham
such
drdha-matih
firmly fixed intelligence
krsne
in Krsna
asit
there was
sangrame ulbane
in the great blazing fire of battle.

Vrtrasura 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 Krsna conscious?