9. The eater is Brahman, because both the movable and immovable universe is taken as His food.
The Katha Upanishad speaks of one for whom the Brahmanas and Kshatriyas are but food and death itself a mere condiment.
The question arises whether this eater is fire, the individual soul, or Brahman.
The sutra establishes that it is Brahman, because the passage describes the reabsorption of the whole creation, movable and immovable, into Him.
The mention of death itself as a condiment shows that even the destroyer of all beings is swallowed up. Therefore the entire universe is referred to as His food.
The Brahmanas and Kshatriyas are mentioned only as examples of the foremost among created beings.
The eater of such a totality can be none other than Brahman.
10. And because Brahman is the subject of the discussion.
Earlier in the Katha Upanishad, Nachiketas asks about that which is beyond good and evil, beyond cause and effect, beyond past and future.
Yama’s reply unfolds as a teaching of the unborn and deathless Self, and the passage concerning the eater appears within this same continuous teaching.
Therefore the eater must be Brahman, since Brahman remains the established topic throughout the discussion.
Another passage says Brahman merely witnesses without eating, but there the word refers to the experience of pleasure and pain. Here it means the reabsorption of the entire universe at dissolution.