Sri Bhagwat Gita: Chapter 2, Verses 18-21
अन्तवन्त इमे देहा नित्यस्योक्ताः शरीरिणः । अनाशिनोऽप्रमेयस्य तस्मात् युध्यस्व भारत ॥
य एनं वेत्ति हन्तारं यश्चैनं मन्यते हतम् । उभौ तौ न विजानीतो नायं हन्ति न हन्यते ॥
न जायते मृयते वा कदाचित् नायं भूत्वा भविता वा न भूयः । अजो नित्यः शाश्वतोयम् पुराणो न हन्यते हन्यमाने शरीरे ॥
वेदा विनाशिनं नित्यं य एनं अजमव्ययं । कथं स पुरुषः पार्थ कम् घातयति हन्ति कम् ॥
Lord Krishna says, those who believe the soul to be destroy-able or those who believe that the soul is capable of killing others are both ignorant; because the reality of soul is that it’s immortal, perpetual and eternal. The soul can neither kill anyone, nor can it be destroyed by any being. The human body that you consider as living is only temporary, it is just the outer shell, and the soul that drives it is permanent.
Lord Krishna addresses Arjun and says the people you are fighting against are already morally and ethically dead due to their bad karmas and poor judgment; you are only doing a formality of physically killing them by holding arms against them. Therefore, hey Arjun, you must at once come out of this perplexity of mind and go for this battle!
Soul is above and beyond the circle of birth and death; its existence can never be reversed and will remain immortal and eternal even if the body that acts as a container for it passes away.
The one who truly understands the permanence and eternity of this soul can never be guilty of or accuse themselves with killing anyone. That’s why O Partha, how can a person who knows that the soul is indestructible, unborn, eternal and immutable, kill anyone or cause anyone to kill?
Here besides describing the un-destructive quality of soul, lord Krishna is also trying to bring home the point that any karma which is done selflessly, with the purity of soul and for the righteous cannot be judged as wrong or sinful.