Mahārāja Yudhiṣṭhira was thinking that, "For me, so many people have been killed. So I have become very, very much sinful. I am not fit for sitting on the throne, such responsible throne." But his misgivings were dissipated by the words of Bhīṣma.
Bhīṣma, he's authority. Next to Kṛṣṇa, Bhīṣma is one of the authorities. There are twelve authorities: svayambhūr nāradaḥ śambhuḥ kapilaḥ kumāro manuḥ, prahlada bhisma janakaḥ (SB 6.3.20). They are the authorities of piety, religion. There are . . . mahājano yena gataḥ sa panthāḥ (CC Madhya 17.186). We cannot decide what is actually piety, religion. But if we follow mahājana, great authorities, then certainly we are unmistaken.