In plain words
One who tormented women must be slain through women themselves. One who takes up the sword is slain by the sword itself. One who takes up the gun is slain by the gun itself. Whatever bad qualities a man has, he is slain by those very bad qualities. This is the law of Prakṛti. Having made Narakāsura weak in this way, Kṛṣṇa did not kill him Himself; He took Satyabhāmā along and had Satyabhāmā kill him. This is the worldly story. But who is that Satyabhāmā? Only envy can kill this anger. Anger alone can kill envy. Between envy and anger there are opposing changes. Who is Satyabhāmā? Envy. If anyone loves Kṛṣṇa, if Kṛṣṇa goes to anyone's house, she will not sleep and will not eat. She keeps burning with that much envy. Kṛṣṇa must be hers alone. He must not belong to anyone else at all. This is the great envy. This envy is not something a person can see. It is like a root-worm. The tree stands there full of life. But this root-worm gets into the root and brings the tree down in a single day. In just the same way, Satyabhāmā is the embodiment of envy in Satya. As to what kind of envy she had, here is another example. Rādhā was Kṛṣṇa's very life, and Kṛṣṇa was Rādhā's life. Satyabhāmā herself would never utter the name Rādhā. While He was talking with her about something, Kṛṣṇa would ask her, ‘Did you go there and come back?’ Kṛṣṇa knew everything. But the director of the play of līlā would ask, ‘Where?’ There! Where? The question again. There! At last He told her to say the name of the one she had been to see. But Satyabhāmā had no wish to say that name. Instead of saying the name straight, she would say it turned round, as ‘Dhāra’. Turn that Dhāra round and it becomes Rādhā. She would not say the name properly. Such was Satyabhāmā's envious mind. So Kṛṣṇa resolved to wipe out this envy. When Satya, His own, Satya, His other half, lived with envy, Kṛṣṇa's very name and fame would be spoiled.