त्रिपुरधर्मवर्णनं नाम तृतीयोऽध्यायः
Description of Dharma in the three cities – Chapter 3
Lord Vishnu said (to the Devas)
इदं सत्यं वचश्चैव यत्र धर्म सनातनः |
तत्र दुःखं न जायेत सूर्ये दृष्टे यथा तमः || 13 ||
This saying is true that where there is Sanatana Dharma, no sorrow/unhappiness emerges, like darkness when the sun is seen.
There was Sanatana Dharma in the three cities of the demons, so everyone there was happy. Lord Brahma, Lord Vishnu or the Great God would not want to kill them.