Karna wasnt really involved in poisining Bhima ( as someone claimed earlier )
"Seeing these wonderful exhibitions of the might of Bhima, the powerful Duryodhana, the eldest son of Dhritarashtra, began to conceive hostility towards him. And the wicked and unrighteous Duryodhana, through ignorance and ambition, prepared himself for an act of sin. He thought, 'There is no other individual who can compare with Bhima, the second son of Pandu, in point of prowess. I shall have to destroy him by artifice. Singly, Bhima dares a century of us to the combat. Therefore, when he shall sleep in the garden, I shall throw him into the current of the Ganga. Afterwards, confining his eldest brother Yudhishthira and his younger brother Arjuna, I shall reign sole king without molestation."