Factually no sin whatsoever touched the Pandavas, who were only the order carriers of the Lord. For others, who declare war out of personal interest, the whole responsibility lies on them
SB Canto 1
I have killed many friends of women, and I have thus caused enmity to such an extent that it is not possible to undo it by material welfare work.
The gṛhamedhīs are those whose only business is to perform welfare work for the sake of material prosperity. Such material prosperity is sometimes hampered by sinful activities, for the materialist is sure to commit sins, even unintentionally, in the course of discharging material duties. To get relief from such sinful reactions, the Vedas prescribe several kinds of sacrifices. It is said in the Vedas that by performing the Aśvamedha-yajña (horse sacrifice) one can get relief from even brahma-hatyā (killing of a brāhmaṇa).
Yudhiṣṭhira Mahārāja performed this Aśvamedha-yajña, but he thinks that even by performing such yajñas it is not possible to get relief from the great sins he committed by killing the friends and relatives of innocent women. In war either a woman's husband or brother or even father or sons go to fight. And when they are killed, a fresh enmity is created, and thus a chain of actions and reactions increases which is not possible to be counteracted even by thousands of Aśvamedha-yajñas.
The way of work (karma) is like that. It creates one action and another reaction simultaneously and thus increases the chain of material activities, binding the performer in material bondage. In the Bhagavad-gītā (Bg. 9.27-28) the remedy is suggested that such actions and reactions of the path of work can be checked only when work is done on behalf of the Supreme Lord. The Battle of Kurukṣetra was actually fought by the will of the Supreme Lord Śrī Kṛṣṇa, as it is evident from His version, and only by His will was Yudhiṣṭhira placed on the throne of Hastināpura. Therefore, factually no sin whatsoever touched the Pāṇḍavas, who were only the order carriers of the Lord. For others, who declare war out of personal interest, the whole responsibility lies on them.