Text 36

papam evasrayed asman
hatvaitan atatayinah
tasman narha vayam hantum
dhartarastran sa-bandhavan
sva-janam hi katham hatva
sukhinah syama madhava

Translation

Sin will overcome us if we slay such aggressors. Therefore it is not proper for us to kill the sons of Dhritarashtra and our friends. What should we gain, O Krishna, husband of the goddess of fortune, and how could we be happy by killing our own kinsmen? 

Commentary by Srila Prabhupada 

According to Vedic injunctions there are six kinds of aggressors: (1) a poison giver, (2) one who sets fire to the house, (3) one who attacks with deadly weapons, (4) one who plunders riches, (5) one who occupies another’s land, and (6) one who kidnaps a wife. Such aggressors are at once to be killed, and no sin is incurred by killing such aggressors. Such killing of aggressors is quite befitting any ordinary man, but Arjuna was not an ordinary person. He was saintly by character, and therefore he wanted to deal with them in saintliness. This kind of saintliness, however, is not for a kshatriya. Although a responsible man in the administration of a state is required to be saintly, he should not be cowardly. For example, Lord Rama was so saintly that people even now are anxious to live in the kingdom of Lord Rama (rama-rajya), but Lord Rama never showed any cowardice. Ravana was an aggressor against Rama because Ravana kidnapped Rama’s wife, Sita, but Lord Rama gave him sufficient lessons, unparalleled in the history of the world. In Arjuna’s case, however, one should consider the special type of aggressors, namely his own grandfather, own teacher, friends, sons, grandsons, etc. Because of them, Arjuna thought that he should not take the severe steps necessary against ordinary aggressors. Besides that, saintly persons are advised to forgive. Such injunctions for saintly persons are more important than any political emergency. Arjuna considered that rather than kill his own kinsmen for political reasons, it would be better to forgive them on grounds of religion and saintly behavior. He did not, therefore, consider such killing profitable simply for the matter of temporary bodily happiness. After all, kingdoms and pleasures derived therefrom are not permanent, so why should he risk his life and eternal salvation by killing his own kinsmen? Arjuna’s addressing of Krishna as “Madhava,” or the husband of the goddess of fortune, is also significant in this connection. He wanted to point out to Krishna that, as husband of the goddess of fortune, He should not induce Arjuna to take up a matter which would ultimately bring about misfortune. Krishna, however, never brings misfortune to anyone, to say nothing of His devotees.

Commentary by Sri Visvanatha Cakravarti Thakur

But it is said:

agnido garadas caiva sastra-panir dhanapahah ksetra-darapahari ca sad ete hy atatayinah

The arsonist, one who poisons, one who attacks with weapons, the thief, the stealer of property and the stealer of ones wife are considered aggressors. Vasistha Smrti 3.19

And also it is said:

atatayinam ayantarh hanyad evavicarayan natatayi-vadhe doso hantur bhavati kascana

Without consideration, one should kill the aggressors, as there is no fault in killing them. Manu Smrti 8.350

Thus the scriptures prescribe killing in the case of aggressors.

Arjuna answers with this verse. Killing them, we will remain, but we will be sinful. The above instructions are from artha sastra, but those instructions are weaker than those from dharma sastra. Yajnavalkya says:

smrtyor virodhe nyayas tu balavan vyavaharatah artha-sastrat tu balavan dharma-sastram iti sthitih

It is established that where there is conflict of rules in two smrti statements, reasoning must prevail of customs to choose the correct rule. However, in reasoning, the rules of dharma sastra are stronger than those of artha sastra. Yajnavalkya Smrti 2.21

Thus though they are aggressors they are also acaryas. In killing acaryas, we will incur sin. We cannot also be happy in this life or the next, since the act is against the rules of dharma and against the conclusion of reasoning. Thus he says "How can we be happy, having killed our own people?"