When under the shroud of delusion, the intelligence considers the ‘wrong’ as ‘right’, and perceives everything contrary to what it is, then that intelligence is tāmasa.
When tamo-guna dominates, one’s thinking becomes totally perverted. Distortion misleads the mind and intelligence right from the start, and one regards the wrong as right. All the arguments follow this initial jumbling.
Karna’s behaviour right from the beginning exemplifies this kind of perversion. His appearance during the Hastinapura princes’ archery display, which Drona, their teacher, had arranged after their tuition was complete, was uncalled for. His going to Parashurama for learning archery, not disclosing his identity, was another tāmasa trait. To speak lies and seek truthful tuition from a great teacher is the gravest travesty one can think of.
Karna, having done nothing to deserve it, accepted the Angarajya crown from Duryodhana. Feeling obligated to Duryodhana ever since, calling him the most ardent friend, indulging in severest wrongs and excesses for his sake, show how tamo-guna overpowers one’s intelligence, landing him into grave crime and torment.