The pious King Yudhiṣṭhira was mortified because of the mass massacre of human beings in the Battle of Kurukṣetra, especially on his account. Duryodhana was there on the throne, and he was doing well in his administration, and in one sense there was no need of fighting. But on the principle of justice Yudhiṣṭhira was to replace him. The whole clique of politics centered around this point, and all the kings and residents of the whole world became involved in this fight between the rival brothers. Lord Kṛṣṇa was also there on the side of King Yudhiṣṭhira. It is said in the Mahābhārata, Adi-parva (20) that 640,000,000 men were killed in the eighteen days of the Battle of Kurukṣetra, and some hundreds of thousands were missing. Practically this was the greatest battle in the world within five thousand years.
This mass killing simply to enthrone Mahārāja Yudhiṣṭhira was too mortifying, so he tried to be convinced with evidences from histories by great sages like Vyāsa and the Lord Himself that the fight was just because the cause was just. But Mahārāja Yudhiṣṭhira would not be satisfied, even though he was instructed by the greatest personalities of the time. Kṛṣṇa is designated herein as the performer of superhuman actions, but in this particular instance neither He nor Vyāsa could convince King Yudhiṣṭhira. Does it mean that He failed to be a superhuman actor? No, certainly not. The interpretation is that the Lord as īśvara, or the Supersoul in the hearts of both King Yudhiṣṭhira and Vyāsa, performed still more superhuman action because the Lord desired it. As Supersoul of King Yudhiṣṭhira, He did not allow the King to be convinced by the words of Vyāsa and others, including Himself, because He desired that the King hear instructions from the dying Bhīṣmadeva, who was another great devotee of the Lord. The Lord wanted that at the last stage of his material existence the great warrior Bhīṣmadeva see Him personally and see his beloved grandchildren, King Yudhiṣṭhira, etc., now situated on the throne, and thus pass away very peacefully. Bhīṣmadeva was not at all satisfied to fight against the Pāṇḍavas, who were his beloved fatherless grandchildren. But the kṣatriyas are also very stern people, and therefore he was obliged to take the side of Duryodhana because he was maintained at the expense of Duryodhana. Besides this, the Lord also desired that King Yudhiṣṭhira be pacified by the words of Bhīṣmadeva so that the world could see that Bhīṣmadeva excelled all in knowledge, including the Lord Himself.