Kṛṣṇa has got two businesses. One, to give protection to the devotee, and the other is to kill the demon. Just like Kṛṣṇa appeared as keśava dhṛta narahari-rūpa (Śrī Daśāvatāra Stotra 4), Nṛsiṁha-deva, and Narahari, Nṛsiṁha-deva, one side He was blessing Prahlāda Mahārāja, the devotee, and the other side He was killing his atheist father, Hiraṇyakaśipu. You have seen the picture. So Kṛṣṇa, when He has got the fighting spirit . . .
Because Kṛṣṇa is the source of everything, janmādy asya yataḥ (SB 1.1.1). Here in this world also, you find two things: friendship and hostility. Just like modern political atmosphere, sometimes there is conference of friendship, and sometimes there is fight with lethal weapons. So this duality is visible in this material world, but in the spiritual world there is no duality. There is absolute peace and . . . (break).
So when He wants to fight, He comes here, because here there are many demons, so His fighting spirit is satisfied. Otherwise, for killing the demons, Kṛṣṇa does not require to descend. There are many agents of Kṛṣṇa. For example, this city, Visakhapatnam, on the seashore, if Kṛṣṇa desires, this whole city can be killed in a second, simply by overflooding the sea, or by storm. He has got many agents, parasya śaktir vividhaiva śruyate (Śvetāśvatara Upaniṣad 6.8, CC Madhya 13.65, purport). Or the material agent, Durgā devī, she can all . . . do all these things.
So Kṛṣṇa does not require to descend for killing the demons, but when He feels . . . just like a rich man sometimes feels to contest with some wrestler, that does not mean he requires it. That is the Kṛṣṇa spirit. He is all-blissful. So when Kṛṣṇa comes, descends, there are many demons. He especially comes for the devotee. But when He fights with the demon, that is also His favor, because anyone killed by Kṛṣṇa, he immediately attains liberation. Therefore Kṛṣṇa is absolute. Either He fights or blesses, the result is the same.