In plain words
The day on which Rāma slew Rāvaṇa, king of the demons, and on that same day was crowned in Ayodhyā, is this Vijayadaśamī. The day on which Vikramāditya ascended the throne, a great gift of grace, is this Vijayadaśamī. The day on which Mahāvīra, founder of the Jaina faith, died is this Vijayadaśamī. In the very same way, the Bhāratīyas have regarded the days on which the wicked died and the days on which the good were born as festivals, and have rejoiced in them. After Kṛṣṇa slew Kaṃsa, the embodiment of adharma, who acted with injustice, wrongdoing and evil conduct, and gave that kingdom to others to rule, Narakāsura, enraged at this, came marching against this Mathurā many times over. Because he marched against Mathurā many times, a kind of fear and panic grew among the people; and to bring peace and security to all the people of Mathurā, He built a town in Dvārakā and settled all these people in it.
The town ruled by this demon Narakāsura was named Prāgjyotiṣāpura. There are four words here. Prāg, jyoti, ṣā, puram. When we grasp the right meaning of these four words, this Prāgjyotiṣāpura makes clear, beautifully, man's very heart. Prāg means 'before', that is, 'earlier'. Jyoti means 'lamp'. Ṣā means 'forgetting'; puram means 'the body'. It is because he forgets the light that was there before, the light of the Ātma, the Supreme Light, the light of Sat-Cit-Ānanda, that man lives within what is called Prāgjyotiṣāpura. Here, in this heart, in this Prāgjyotiṣāpura, this naratva called the demonic has utterly forgotten humanness. Naruḍu means the Ātma; that is its meaning. Naratva is the very place from which the elements, space and the rest, came forth. In this naratva, because of the elements, space and the rest, they took it to be full of water and so went on calling it nāra as well. Nāras means living beings: that is another meaning. Because He is the abode of the nāras, He came by the name Nārāyaṇa. What does rahaḥ mean? Na means 'not'. Rahaḥ also gives the meaning 'permanent'; nara means 'not permanent': that is the meaning. Humanness is not what is eternal and true. When the evil qualities called the demonic enter into this city called nara, he himself becomes Narakāsura. Evil qualities are the very marks of the demonic.
So, because the two principles, nara and asura, came together, he became Narakāsura. Narakāsura also has another meaning: one who leads to hell. But when we look into the story of the Narakāsura of history, he was utterly wicked and evil. In that kingdom no lamp was ever lit, in the bazaar or in the home. In that kingdom no one could be seen by anyone. They had never seen the light of a lamp. So wicked was this Narakāsura.