Karna's downfall in the Mahabharata war was due to multiple factors: Bhoomi Devi's curse meant his chariot wheel would get stuck (which happened at the crucial moment), he had promised his mother Kunti he wouldn't use divine weapons twice against Arjun, Krishna saved Arjun twice during their duel, and Karna's role in killing Abhimanyu violated the war principles established by Bhishma. All these factors combined led to his death at Arjun's hands.