QUESTION: Bhisma is considered as one of the 12 Mahajana, which means he is a pure Vaisanava, so how come he fought against the Pandavas on the side of the adharmi?
He made a vow to protect the throne of Hastinapura, so he was bound by that vow. To break it would be adharma. At the same time, to fight on the side of the adharmi Duryodhana is also adharma, so he did not really fight on that side. He fought but did not put his heart into the fight, in fact, he told Arjuna the secret about how to defeat him, and thus gave Arjuna the secret to defeating Duryodhana.
Thus he found a way to not break his vow, but also not to be of service to an adharmi.
QUESTION: But when Arjuna kills Bhisma, a Vaisanava kills another Vaisanava. Won’t this be a Vaisanava Apradh? And can this be used by splinter groups in a Parampara to justify their opposition to one another?
These two Vaiṣṇava’s were kṣatriyas performing their duty. Moreover, Arjuna did this only with Bhiṣma’s permission. Therefore it is not aparadha and it is nothing like the infighting that goes on between religious institutions envious of one another’s resources.
Vraja Kishor das