Verse 25: Avyakta, Achintya, Avikarya Avikriya: This Self is unmanifest as it is not an object to all senses including the mind and intellect. Therefore it is beyond thought. That which can be perceived by the senses is alone object for thought. This Self is not apprehended by the sense organs of perception and action. So it cannot be thought about. As the milk gets transformed into curd by dropping a little bit of curd into it. this Self is not transformed in that manner.
Since there are no parts, the Self is without any modification. Nowhere has something without parts seem to show any modification. Since it does not have any changes, the Self is called unmodifying and unchanging.
Therefore, as mentioned in this manner, having known the Self, it does not befit you to worry saying, “I am the killer of these people and these people are killed by me.” Accepting the impermanence of the Self, Bhagavan says in the next verse:

Verse 26: Eternally born: The two indeclinables, Atha and cha – then and and, are used for the purpose of denoting acceptance of the idea of impermanence. Bhagavan says, ” If you think that the Atman spoken of in this manner is ever born as seen popularly in the world, the life which springs forth in all beings in this world is considered to be born, or when each being dies everyday, it is termed as dead, the person is called a dead body, even in that case O broad-shouldered Arjuna, when this is the case, that which is born will die and that which dies will be born, this is a necessary reality thus it does not befit you to worry so about Self.

– Brahmacharini Sharanya Chaitanya.