So these kings were meant for giving protection to the citizens, to train them to the Vedic conception of life. But they were not doing that. Therefore they were demons.
Although they were kṣatriyas, still... Kṣatriyas are meant for representing the Supreme Personality of Godhead. Actually, Kṛṣṇa is the proprietor. Sarva-loka-maheśvaram. Sarva-loka-maheśvaram, suhṛdaṁ sarva-bhūtānāṁ jñātvā māṁ śāntim ṛcchati (BG 5.29). So He is the proprietor. Sarva-loka. Sarva-loka means all the planets, all the universes. Because they are created by Kṛṣṇa, by Kṛṣṇa's bodily effulgence, brahma-jyotir. So they are created by Kṛṣṇa's. Just like from the sunshine these planets are created, similarly from brahma-jyotir, innumerable universes are created.