B.G 2.21

वेदाविनाशिनं नित्यं य एनमजमव्ययम् । कथं स पुरुषः पार्थ कं घातयति हन्ति कम् ॥ २-२१॥

vēdāvināśinaṁ nityaṁ ya ēnamajamavyayam । kathaṁ sa puruṣaḥ pārtha kaṁ ghātayati hanti kam ॥ 2-21॥

[vēda = one who knows; avināśinaṁ = the indestructible; nityaṁ = always existing as essence without deformities; ya = he who; ēnamajamavyayam = this One, the Lord, is unborn, unchangable; kathaṁ = how; sa = that; puruṣaḥ = Jīva; pārtha = O Partha; kaṁ ghātayati = How can he hurt; hanti = kills; kam = whom;]

O Arjuna, how can the Jīva who knows thus the Indestructible Lord to be (the doer), ever existing, unborn and unchangeable, think of being killed or hurting another?

Gīta Bhāshya 2.21

The words 'avinashina' and 'nitya' refer to the Lord. Word 'nashtha' is commonly used to address Jīva, as it means tainted with defect. The word 'Vēda' refers to the imparted knowledge till previous verse, that the Lord is the doer of all acts and Jīva is not an independent agent.

अतो य एवं वेद स कथं कं घातयति हन्ति वा। अविनाशिनं नैमित्तिकनाशरहितम्। नित्यं स्वाभाविकनाशरहितम् अथवा अविनाशिनं दोषयोगरहितम्। नित्यं सदा भाविनम्। इति सर्वत्र विवेकः । दोषयुक्त पुरुषादिषु नष्टशब्दप्रयोगात्॥ २१ ॥

atō ya ēvaṁ vēda sa kathaṁ kaṁ ghātayati hanti vā। avināśinaṁ naimittikanāśarahitam। nityaṁ svābhāvikanāśarahitam athavā avināśinaṁ dōṣayōgarahitam। nityaṁ sadā bhāvinam। iti sarvatra vivēkaḥ । dōṣayukta puruṣādiṣu naṣṭaśabdaprayōgāt॥ 21 ॥

[atō ya ēvaṁ vēda = one who understands thus; sa kathaṁ =How can; kaṁ ghātayati hanti vā = think of killing or causing another to kill?; avināśinaṁ naimittikanāśarahitam = "avinâshina" - means one who does not have a destruction of the form; nityaṁ svābhāvikanāśarahitam = "nitya" - means one who does not have a destruction of original nature;athavā = or; avināśinaṁ dōṣayōgarahitam = "avinâshina" - means one who is untouched by defects; nityaṁ sadā bhāvinam = "nitya" - means one who exists always; iti sarvatra vivēkaḥ = Above said is common wisdom. । dōṣayukta puruṣādiṣu naṣṭaśabdaprayōgāt = As Jîva is tainted with defects, it is addressed by the word 'nashta'; ]

How can one who understands thus (Jîva to be eternal and dependent) think of killing or causing another to kill?

"avinâshina" - means one who does not have a destruction of the form; and "nitya" - means one who does not have a destruction of original nature;

or

"avinâshina" - means one who is untouched by defects, and "nitya" - means one who exists always.

Above said is common wisdom.

As Jîva is tainted with defects, the word 'nashta' is used to address it.

Gīta Tātparya 2.21

'avinashina' indicates lord without destruction of his body, 'nitya' indicates non-variance in his essential nature. The agency must always primarily be attributed to the Lord and only the ignorant attribute it to themselves.

अविनाशिनं शरीरापायादिवर्जितम्। नित्यं स्वरूपतः। एनं परमेश्वरम्।

"कर्तृत्वं तु स्वतन्त्रत्वं तदेकस्य हरेर्भवेत् ।तच्चाव्ययं तस्य जानन् कथं कर्ता स्वयं भवेत्" - इति परमश्रुतिः।

अन्यथा अविनाशिनं नित्यमिति पुनरुक्तिः ॥२१ ॥

avināśinaṁ śarīrāpāyādivarjitam। nityaṁ svarūpataḥ। ēnaṁ paramēśvaram।

"kartr̥tvaṁ tu svatantratvaṁ tadēkasya harērbhavēt ।taccāvyayaṁ tasya jānan kathaṁ kartā svayaṁ bhavēt" - iti paramaśrutiḥ।

anyathā avināśinaṁ nityamiti punaruktiḥ ॥21 ॥

[avināśinaṁ śarīrāpāyādivarjitam = 'avinashinam' indicates one who is devoid of defects such as destruction of the body; nityaṁ svarūpataḥ = 'nityam' indicates ones own essense;। ēnaṁ paramēśvaram = 'enam' is refered to the supreme lord; ]

[kartr̥tvaṁ tu svatantratvaṁ = the notion of agency is nothing but freedom to do; tadēkasya harērbhavēt = it exists in Lord Hari alone; taccāvyayaṁ tasya = it is present in Him always, without destruction; jānan kathaṁ kartā svayaṁ bhavēt = How can one with this understanding think of himself as doer?" - iti paramaśrutiḥ = thus states Parama sruti; ]

The word 'avinashinam' indicates one who is devoid of defects such as the destruction of the body; 'nityam' indicates one's own essence; 'enam' is a reference to the supreme lord.

"The notion of agency is nothing but freedom to do. It exists in Lord Hari alone. It is present in Him always, without destruction. How can one with this understanding think of himself as a doer?" - thus states Parama sruti

If not interpreted this way, the words 'avinashina' and 'nitya' providing same meaning results in redundancy error.