Question: Why did Bhismadeva decline to fight on the same side as Karna?
Answer by Romapada Swami:
External reason:
• Bhisma viewed Karna as unqualified, due to being excessively boastful, disrespectful, plus inferior, famously labeling him a “half-ratha” (half-chariot warrior).
• E.g. Karna boasted that he alone will kill all the Pandavas. Elaborately, with many examples, Bhisma dismissed Karna’s boastful claim, then mocked him saying that Krishna is protecting the Pandavas. Karna became angry and declared that he no longer will fight as long as Bhisma was on the battlefield.
Confidential unspoken ethical reason:
• Bhisma knew through Narada and Krishna that Karna was Kunti’s first son. He did not want to see a fight to the death between the eldest Pandava and his younger brothers.
