Katha-1-2-19-hantā cenmanyate

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Even the âtman of such description, the slayer who sees the mere body as the âtman thinks of slaying and he who thinks that his âtman is slain, both these do not know their own âtman; for, he does not slay the âtman, being incapable of modification; nor is he slain being incapable of modification like the âkâs. Therefore, all samsâra, the fruit of virtue and vice is only in the case of those who do not know the âtman, and not in the case of one who knows the Brahman; for in his case, virtue and vice are inappropriate both from the authority of the srutis and from the cogency of reasoning.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda