Those who know that the soul is eternal do not attribute changes in the body to the soul.

Those who know that the soul is immortal and eternal, unborn, and undiminishing, how can they kill or cause someone to be killed? Krishna asks this question.

The fact that the event of killing occurs in the physical world is not denied. What is distinguished here is the understanding that this act is attributed to the immutable soul itself. This section should not be interpreted as negating responsibility for the action, but rather the main point is about the immortality of the soul. Those who know the soul do not see bodily death as the extinction of existence itself, and they do not confuse actions performed through the body with the nature of the soul.

Japanese Translation from the Geeta Press Edition - Chapter 2, Section 21: "Who Kills Whom?"

In section 19, the Lord stated that no one kills the soul, and the soul is not killed by anyone. Following this, in section 20, it is shown that the soul transcends change, and why the soul cannot be killed by anyone is revealed.

In the following section, it will be explained why the soul does not kill anyone.

Sanskrit original text

वेदाविनाशिनं नित्यं य एनमजमव्ययम् ।
कथं स पुरुषः पार्थ कं घातयति हन्ति कम् ॥ २१ ॥

Romanization

vedāvināśinaṃ nityaṃ ya enam ajam avyayam |
kathaṃ sa puruṣaḥ pārtha kaṃ ghātayati hanti kam || 21 ||

Arjuna, know that the one who understands this soul to be immortal and eternal, beyond birth and decay, how can he kill anyone or be killed by anyone? How can he kill anyone or be killed by anyone? (21)

In this section, the author presents the idea that those who truly understand the nature of the soul would never consider killing someone themselves or having someone else killed.

In other words, that person knows that what is killed is the body, and even that body has a mind, intelligence, and senses.

Then, how can one identify oneself with the body, and believe that they have killed someone, or caused someone to be killed?

In that person's consciousness, only a single spirit remains, which exists everywhere. It cannot die or be killed, and it also cannot kill or cause someone else to be killed.

Therefore, actions such as being exposed to death, giving death to others, or causing the death of others are attributed to the soul due to ignorance. In reality, they have nothing to do with the soul. Therefore, it is not reasonable to feel sadness for any existence.