BG 2.21

vedāvināśinaṁ nityaṁ
ya enam ajam avyayam
kathaṁ sa puruṣaḥ pārtha
kaṁ ghātayati hanti kam

Synonyms:

veda — knows; avināśinam — indestructible; nityam — always existing; yaḥ — one who; enam — this (soul); ajam — unborn; avyayam — immutable; katham — how; saḥ — that; puruṣaḥ — person; pārtha — O Pārtha (Arjuna); kam — whom; ghātayati — causes to hurt; hanti — kills; kam — whom.

Translation:

O Pārtha, how can a person who knows that the soul is indestructible, eternal, unborn and immutable kill anyone or cause anyone to kill?

vedavinasinam nityam, veda+avinasinam, know that it is avinasi and Nityam

ya enam ajam avyayam, same point we talked unborn and imperishable

katham sa purushah partha, kam ghatayati hanti kam, so it is a rhetorical way of continuing and repeating what is talked in 19th verse, that the person never kills nor it is killed. kam ghatayati hanti kam, so who is the killer and who is killed? Now Atma is eternal and the body is destructible, therefore who is the killer of indestructible soul? No body. The soul cannot be killed; therefore nobody and in this way Sri Krishna is emphatically driving home the point that the soul is eternal.

BG 2.13 -> soul is specific being

And now after talking about (BG 2.21) soul as indestructible from next verse; Sri Krishna will talk about (BG 2.22) how the soul continues to exist even in the next life or next body.